Commit e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing
to VP assist page MSR") moved 'wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE)' under
'if (*hvp)' condition. This works for root partition as hv_cpu_die()
does memunmap() and sets 'hv_vp_assist_page[cpu]' to NULL but breaks
non-root partitions as hv_cpu_die() doesn't free 'hv_vp_assist_page[cpu]'
for them. This causes VP assist page to remain unset after CPU
offline/online cycle:
$ rdmsr -p 24 0x40000073
10212f001
$ echo 0 > /sys/devices/system/cpu/cpu24/online
$ echo 1 > /sys/devices/system/cpu/cpu24/online
$ rdmsr -p 24 0x40000073
0
Fix the issue by always writing to HV_X64_MSR_VP_ASSIST_PAGE in
hv_cpu_init(). Note, checking 'if (!*hvp)', for root partition is
pointless as hv_cpu_die() always sets 'hv_vp_assist_page[cpu]' to
NULL (and it's also NULL initially).
Note: the fact that 'hv_vp_assist_page[cpu]' is reset to NULL may
present a (potential) issue for KVM. While Hyper-V uses
CPUHP_AP_ONLINE_DYN stage in CPU hotplug, KVM uses CPUHP_AP_KVM_STARTING
which comes earlier in CPU teardown sequence. It is theoretically
possible that Enlightened VMCS is still in use. It is unclear if the
issue is real and if using KVM with Hyper-V root partition is even
possible.
While on it, drop the unneeded smp_processor_id() call from hv_cpu_init().
Fixes: e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing to VP assist page MSR")
Signed-off-by: Vitaly Kuznetsov <[email protected]>
---
arch/x86/hyperv/hv_init.c | 54 +++++++++++++++++++--------------------
1 file changed, 26 insertions(+), 28 deletions(-)
diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index f49bc3ec76e6..a269049a43ce 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -77,7 +77,7 @@ static int hyperv_init_ghcb(void)
static int hv_cpu_init(unsigned int cpu)
{
union hv_vp_assist_msr_contents msr = { 0 };
- struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
+ struct hv_vp_assist_page **hvp = &hv_vp_assist_page[cpu];
int ret;
ret = hv_common_cpu_init(cpu);
@@ -87,34 +87,32 @@ static int hv_cpu_init(unsigned int cpu)
if (!hv_vp_assist_page)
return 0;
- if (!*hvp) {
- if (hv_root_partition) {
- /*
- * For root partition we get the hypervisor provided VP assist
- * page, instead of allocating a new page.
- */
- rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
- *hvp = memremap(msr.pfn <<
- HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
- PAGE_SIZE, MEMREMAP_WB);
- } else {
- /*
- * The VP assist page is an "overlay" page (see Hyper-V TLFS's
- * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
- * out to make sure we always write the EOI MSR in
- * hv_apic_eoi_write() *after* the EOI optimization is disabled
- * in hv_cpu_die(), otherwise a CPU may not be stopped in the
- * case of CPU offlining and the VM will hang.
- */
+ if (hv_root_partition) {
+ /*
+ * For root partition we get the hypervisor provided VP assist
+ * page, instead of allocating a new page.
+ */
+ rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
+ *hvp = memremap(msr.pfn << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
+ PAGE_SIZE, MEMREMAP_WB);
+ } else {
+ /*
+ * The VP assist page is an "overlay" page (see Hyper-V TLFS's
+ * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
+ * out to make sure we always write the EOI MSR in
+ * hv_apic_eoi_write() *after* the EOI optimization is disabled
+ * in hv_cpu_die(), otherwise a CPU may not be stopped in the
+ * case of CPU offlining and the VM will hang.
+ */
+ if (!*hvp)
*hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
- if (*hvp)
- msr.pfn = vmalloc_to_pfn(*hvp);
- }
- WARN_ON(!(*hvp));
- if (*hvp) {
- msr.enable = 1;
- wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
- }
+ if (*hvp)
+ msr.pfn = vmalloc_to_pfn(*hvp);
+
+ }
+ if (!WARN_ON(!(*hvp))) {
+ msr.enable = 1;
+ wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
}
return hyperv_init_ghcb();
--
2.38.1
From: Vitaly Kuznetsov <[email protected]> Sent: Thursday, November 3, 2022 12:06 PM
>
> Commit e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing
> to VP assist page MSR") moved 'wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE)' under
> 'if (*hvp)' condition. This works for root partition as hv_cpu_die()
> does memunmap() and sets 'hv_vp_assist_page[cpu]' to NULL but breaks
> non-root partitions as hv_cpu_die() doesn't free 'hv_vp_assist_page[cpu]'
> for them.
Did you consider having hv_cpu_die() free the VP assist page and
set hv_vp_assist_page[cpu] to NULL in the non-root case? That would
make the root and non-root cases more consistent, and it would make
hv_cpu_init() and hv_cpu_die() more symmetrical. The hv_cpu_die()
path frees up pretty much all the other per-CPU resources. I don't
know why it keeps the VP assist page for re-use if the CPU comes back
online later.
You added the original code for allocating the vp_assist_page in
commit a46d15cc1a, so maybe you remember if there are any
gotchas. :-)
Michael
> This causes VP assist page to remain unset after CPU
> offline/online cycle:
>
> $ rdmsr -p 24 0x40000073
> 10212f001
> $ echo 0 > /sys/devices/system/cpu/cpu24/online
> $ echo 1 > /sys/devices/system/cpu/cpu24/online
> $ rdmsr -p 24 0x40000073
> 0
>
> Fix the issue by always writing to HV_X64_MSR_VP_ASSIST_PAGE in
> hv_cpu_init(). Note, checking 'if (!*hvp)', for root partition is
> pointless as hv_cpu_die() always sets 'hv_vp_assist_page[cpu]' to
> NULL (and it's also NULL initially).
>
> Note: the fact that 'hv_vp_assist_page[cpu]' is reset to NULL may
> present a (potential) issue for KVM. While Hyper-V uses
> CPUHP_AP_ONLINE_DYN stage in CPU hotplug, KVM uses
> CPUHP_AP_KVM_STARTING
> which comes earlier in CPU teardown sequence. It is theoretically
> possible that Enlightened VMCS is still in use. It is unclear if the
> issue is real and if using KVM with Hyper-V root partition is even
> possible.
>
> While on it, drop the unneeded smp_processor_id() call from hv_cpu_init().
>
> Fixes: e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing to VP assist
> page MSR")
> Signed-off-by: Vitaly Kuznetsov <[email protected]>
> ---
> arch/x86/hyperv/hv_init.c | 54 +++++++++++++++++++--------------------
> 1 file changed, 26 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index f49bc3ec76e6..a269049a43ce 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -77,7 +77,7 @@ static int hyperv_init_ghcb(void)
> static int hv_cpu_init(unsigned int cpu)
> {
> union hv_vp_assist_msr_contents msr = { 0 };
> - struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
> + struct hv_vp_assist_page **hvp = &hv_vp_assist_page[cpu];
> int ret;
>
> ret = hv_common_cpu_init(cpu);
> @@ -87,34 +87,32 @@ static int hv_cpu_init(unsigned int cpu)
> if (!hv_vp_assist_page)
> return 0;
>
> - if (!*hvp) {
> - if (hv_root_partition) {
> - /*
> - * For root partition we get the hypervisor provided VP assist
> - * page, instead of allocating a new page.
> - */
> - rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> - *hvp = memremap(msr.pfn <<
> -
> HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
> - PAGE_SIZE, MEMREMAP_WB);
> - } else {
> - /*
> - * The VP assist page is an "overlay" page (see Hyper-V TLFS's
> - * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
> - * out to make sure we always write the EOI MSR in
> - * hv_apic_eoi_write() *after* the EOI optimization is disabled
> - * in hv_cpu_die(), otherwise a CPU may not be stopped in the
> - * case of CPU offlining and the VM will hang.
> - */
> + if (hv_root_partition) {
> + /*
> + * For root partition we get the hypervisor provided VP assist
> + * page, instead of allocating a new page.
> + */
> + rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> + *hvp = memremap(msr.pfn <<
> HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
> + PAGE_SIZE, MEMREMAP_WB);
> + } else {
> + /*
> + * The VP assist page is an "overlay" page (see Hyper-V TLFS's
> + * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
> + * out to make sure we always write the EOI MSR in
> + * hv_apic_eoi_write() *after* the EOI optimization is disabled
> + * in hv_cpu_die(), otherwise a CPU may not be stopped in the
> + * case of CPU offlining and the VM will hang.
> + */
> + if (!*hvp)
> *hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
> - if (*hvp)
> - msr.pfn = vmalloc_to_pfn(*hvp);
> - }
> - WARN_ON(!(*hvp));
> - if (*hvp) {
> - msr.enable = 1;
> - wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> - }
> + if (*hvp)
> + msr.pfn = vmalloc_to_pfn(*hvp);
> +
> + }
> + if (!WARN_ON(!(*hvp))) {
> + msr.enable = 1;
> + wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> }
>
> return hyperv_init_ghcb();
> --
> 2.38.1
"Michael Kelley (LINUX)" <[email protected]> writes:
> From: Vitaly Kuznetsov <[email protected]> Sent: Thursday, November 3, 2022 12:06 PM
>>
>> Commit e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing
>> to VP assist page MSR") moved 'wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE)' under
>> 'if (*hvp)' condition. This works for root partition as hv_cpu_die()
>> does memunmap() and sets 'hv_vp_assist_page[cpu]' to NULL but breaks
>> non-root partitions as hv_cpu_die() doesn't free 'hv_vp_assist_page[cpu]'
>> for them.
>
> Did you consider having hv_cpu_die() free the VP assist page and
> set hv_vp_assist_page[cpu] to NULL in the non-root case?
Oh yes, I did, and I even wrote a patch (attached) but it failed in
testing :-( My testing was to run CPU onlining/offlining loop and and
try using KVM on the same CPU at the same time. I was able to see issues
when KVM was not able to reach to Enlightened VMCS because VP assist
page was NULL.
> That would
> make the root and non-root cases more consistent, and it would make
> hv_cpu_init() and hv_cpu_die() more symmetrical. The hv_cpu_die()
> path frees up pretty much all the other per-CPU resources. I don't
> know why it keeps the VP assist page for re-use if the CPU comes back
> online later.
>
> You added the original code for allocating the vp_assist_page in
> commit a46d15cc1a, so maybe you remember if there are any
> gotchas. :-)
The root cause of the problem I observed seems to be the order of CPU
hotplug. Hyper-V uses the last CPUHP_AP_ONLINE_DYN stage while KVM has
his own dedicated CPUHP_AP_KVM_STARTING one so we end up freeing VP
assist page before turning KVM off on the CPU. It may be sufficient to
introduce a new stage for Hyper-V and put it before KVM's. It's in my
backlog to explore this path but it may take me some time to get back to
it :-(
>
> Michael
>
>> This causes VP assist page to remain unset after CPU
>> offline/online cycle:
>>
>> $ rdmsr -p 24 0x40000073
>> 10212f001
>> $ echo 0 > /sys/devices/system/cpu/cpu24/online
>> $ echo 1 > /sys/devices/system/cpu/cpu24/online
>> $ rdmsr -p 24 0x40000073
>> 0
>>
>> Fix the issue by always writing to HV_X64_MSR_VP_ASSIST_PAGE in
>> hv_cpu_init(). Note, checking 'if (!*hvp)', for root partition is
>> pointless as hv_cpu_die() always sets 'hv_vp_assist_page[cpu]' to
>> NULL (and it's also NULL initially).
>>
>> Note: the fact that 'hv_vp_assist_page[cpu]' is reset to NULL may
>> present a (potential) issue for KVM. While Hyper-V uses
>> CPUHP_AP_ONLINE_DYN stage in CPU hotplug, KVM uses
>> CPUHP_AP_KVM_STARTING
>> which comes earlier in CPU teardown sequence. It is theoretically
>> possible that Enlightened VMCS is still in use. It is unclear if the
>> issue is real and if using KVM with Hyper-V root partition is even
>> possible.
>>
>> While on it, drop the unneeded smp_processor_id() call from hv_cpu_init().
>>
>> Fixes: e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing to VP assist
>> page MSR")
>> Signed-off-by: Vitaly Kuznetsov <[email protected]>
>> ---
>> arch/x86/hyperv/hv_init.c | 54 +++++++++++++++++++--------------------
>> 1 file changed, 26 insertions(+), 28 deletions(-)
>>
>> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
>> index f49bc3ec76e6..a269049a43ce 100644
>> --- a/arch/x86/hyperv/hv_init.c
>> +++ b/arch/x86/hyperv/hv_init.c
>> @@ -77,7 +77,7 @@ static int hyperv_init_ghcb(void)
>> static int hv_cpu_init(unsigned int cpu)
>> {
>> union hv_vp_assist_msr_contents msr = { 0 };
>> - struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
>> + struct hv_vp_assist_page **hvp = &hv_vp_assist_page[cpu];
>> int ret;
>>
>> ret = hv_common_cpu_init(cpu);
>> @@ -87,34 +87,32 @@ static int hv_cpu_init(unsigned int cpu)
>> if (!hv_vp_assist_page)
>> return 0;
>>
>> - if (!*hvp) {
>> - if (hv_root_partition) {
>> - /*
>> - * For root partition we get the hypervisor provided VP assist
>> - * page, instead of allocating a new page.
>> - */
>> - rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> - *hvp = memremap(msr.pfn <<
>> -
>> HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
>> - PAGE_SIZE, MEMREMAP_WB);
>> - } else {
>> - /*
>> - * The VP assist page is an "overlay" page (see Hyper-V TLFS's
>> - * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
>> - * out to make sure we always write the EOI MSR in
>> - * hv_apic_eoi_write() *after* the EOI optimization is disabled
>> - * in hv_cpu_die(), otherwise a CPU may not be stopped in the
>> - * case of CPU offlining and the VM will hang.
>> - */
>> + if (hv_root_partition) {
>> + /*
>> + * For root partition we get the hypervisor provided VP assist
>> + * page, instead of allocating a new page.
>> + */
>> + rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> + *hvp = memremap(msr.pfn <<
>> HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
>> + PAGE_SIZE, MEMREMAP_WB);
>> + } else {
>> + /*
>> + * The VP assist page is an "overlay" page (see Hyper-V TLFS's
>> + * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
>> + * out to make sure we always write the EOI MSR in
>> + * hv_apic_eoi_write() *after* the EOI optimization is disabled
>> + * in hv_cpu_die(), otherwise a CPU may not be stopped in the
>> + * case of CPU offlining and the VM will hang.
>> + */
>> + if (!*hvp)
>> *hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
>> - if (*hvp)
>> - msr.pfn = vmalloc_to_pfn(*hvp);
>> - }
>> - WARN_ON(!(*hvp));
>> - if (*hvp) {
>> - msr.enable = 1;
>> - wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> - }
>> + if (*hvp)
>> + msr.pfn = vmalloc_to_pfn(*hvp);
>> +
>> + }
>> + if (!WARN_ON(!(*hvp))) {
>> + msr.enable = 1;
>> + wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> }
>>
>> return hyperv_init_ghcb();
>> --
>> 2.38.1
>
--
Vitaly
From: Vitaly Kuznetsov <[email protected]> Sent: Thursday, November 3, 2022 12:06 PM
>
> Commit e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing
> to VP assist page MSR") moved 'wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE)' under
> 'if (*hvp)' condition. This works for root partition as hv_cpu_die()
> does memunmap() and sets 'hv_vp_assist_page[cpu]' to NULL but breaks
> non-root partitions as hv_cpu_die() doesn't free 'hv_vp_assist_page[cpu]'
> for them. This causes VP assist page to remain unset after CPU
> offline/online cycle:
>
> $ rdmsr -p 24 0x40000073
> 10212f001
> $ echo 0 > /sys/devices/system/cpu/cpu24/online
> $ echo 1 > /sys/devices/system/cpu/cpu24/online
> $ rdmsr -p 24 0x40000073
> 0
>
> Fix the issue by always writing to HV_X64_MSR_VP_ASSIST_PAGE in
> hv_cpu_init(). Note, checking 'if (!*hvp)', for root partition is
> pointless as hv_cpu_die() always sets 'hv_vp_assist_page[cpu]' to
> NULL (and it's also NULL initially).
>
> Note: the fact that 'hv_vp_assist_page[cpu]' is reset to NULL may
> present a (potential) issue for KVM. While Hyper-V uses
> CPUHP_AP_ONLINE_DYN stage in CPU hotplug, KVM uses
> CPUHP_AP_KVM_STARTING
> which comes earlier in CPU teardown sequence. It is theoretically
> possible that Enlightened VMCS is still in use. It is unclear if the
> issue is real and if using KVM with Hyper-V root partition is even
> possible.
>
> While on it, drop the unneeded smp_processor_id() call from hv_cpu_init().
>
> Fixes: e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing to VP assist
> page MSR")
> Signed-off-by: Vitaly Kuznetsov <[email protected]>
> ---
> arch/x86/hyperv/hv_init.c | 54 +++++++++++++++++++--------------------
> 1 file changed, 26 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index f49bc3ec76e6..a269049a43ce 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -77,7 +77,7 @@ static int hyperv_init_ghcb(void)
> static int hv_cpu_init(unsigned int cpu)
> {
> union hv_vp_assist_msr_contents msr = { 0 };
> - struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
> + struct hv_vp_assist_page **hvp = &hv_vp_assist_page[cpu];
> int ret;
>
> ret = hv_common_cpu_init(cpu);
> @@ -87,34 +87,32 @@ static int hv_cpu_init(unsigned int cpu)
> if (!hv_vp_assist_page)
> return 0;
>
> - if (!*hvp) {
> - if (hv_root_partition) {
> - /*
> - * For root partition we get the hypervisor provided VP assist
> - * page, instead of allocating a new page.
> - */
> - rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> - *hvp = memremap(msr.pfn <<
> -
> HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
> - PAGE_SIZE, MEMREMAP_WB);
> - } else {
> - /*
> - * The VP assist page is an "overlay" page (see Hyper-V TLFS's
> - * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
> - * out to make sure we always write the EOI MSR in
> - * hv_apic_eoi_write() *after* the EOI optimization is disabled
> - * in hv_cpu_die(), otherwise a CPU may not be stopped in the
> - * case of CPU offlining and the VM will hang.
> - */
> + if (hv_root_partition) {
> + /*
> + * For root partition we get the hypervisor provided VP assist
> + * page, instead of allocating a new page.
> + */
> + rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> + *hvp = memremap(msr.pfn <<
> HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
> + PAGE_SIZE, MEMREMAP_WB);
> + } else {
> + /*
> + * The VP assist page is an "overlay" page (see Hyper-V TLFS's
> + * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
> + * out to make sure we always write the EOI MSR in
> + * hv_apic_eoi_write() *after* the EOI optimization is disabled
> + * in hv_cpu_die(), otherwise a CPU may not be stopped in the
> + * case of CPU offlining and the VM will hang.
> + */
> + if (!*hvp)
> *hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
> - if (*hvp)
> - msr.pfn = vmalloc_to_pfn(*hvp);
> - }
> - WARN_ON(!(*hvp));
> - if (*hvp) {
> - msr.enable = 1;
> - wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> - }
> + if (*hvp)
> + msr.pfn = vmalloc_to_pfn(*hvp);
> +
> + }
> + if (!WARN_ON(!(*hvp))) {
> + msr.enable = 1;
> + wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> }
>
> return hyperv_init_ghcb();
> --
> 2.38.1
Reviewed-by: Michael Kelley <[email protected]>
On Thu, Nov 03, 2022 at 08:06:01PM +0100, Vitaly Kuznetsov wrote:
> Commit e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing
> to VP assist page MSR") moved 'wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE)' under
> 'if (*hvp)' condition. This works for root partition as hv_cpu_die()
> does memunmap() and sets 'hv_vp_assist_page[cpu]' to NULL but breaks
> non-root partitions as hv_cpu_die() doesn't free 'hv_vp_assist_page[cpu]'
> for them. This causes VP assist page to remain unset after CPU
> offline/online cycle:
>
> $ rdmsr -p 24 0x40000073
> 10212f001
> $ echo 0 > /sys/devices/system/cpu/cpu24/online
> $ echo 1 > /sys/devices/system/cpu/cpu24/online
> $ rdmsr -p 24 0x40000073
> 0
>
> Fix the issue by always writing to HV_X64_MSR_VP_ASSIST_PAGE in
> hv_cpu_init(). Note, checking 'if (!*hvp)', for root partition is
> pointless as hv_cpu_die() always sets 'hv_vp_assist_page[cpu]' to
> NULL (and it's also NULL initially).
>
> Note: the fact that 'hv_vp_assist_page[cpu]' is reset to NULL may
> present a (potential) issue for KVM. While Hyper-V uses
> CPUHP_AP_ONLINE_DYN stage in CPU hotplug, KVM uses CPUHP_AP_KVM_STARTING
> which comes earlier in CPU teardown sequence. It is theoretically
> possible that Enlightened VMCS is still in use. It is unclear if the
> issue is real and if using KVM with Hyper-V root partition is even
> possible.
>
> While on it, drop the unneeded smp_processor_id() call from hv_cpu_init().
>
> Fixes: e5d9b714fe40 ("x86/hyperv: fix root partition faults when writing to VP assist page MSR")
> Signed-off-by: Vitaly Kuznetsov <[email protected]>
Applied to hyperv-fixes. Thanks.