From: Joerg Roedel <[email protected]>
> Sent: 26 March 2022 14:41
>
> The io specific memcpy/memset functions use string mmio accesses to do
> their work. Under SEV the hypervisor can't emulate these instructions,
> because they read/write directly from/to encrypted memory.
>
> KVM will inject a page fault exception into the guest when it is asked
> to emulate string mmio instructions for an SEV guest:
>
> BUG: unable to handle page fault for address: ffffc90000065068
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 8000100000067 P4D 8000100000067 PUD 80001000fb067 PMD 80001000fc067 PTE 80000000fed40173
> Oops: 0000 [#1] PREEMPT SMP NOPTI
> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.17.0-rc7 #3
>
> As string mmio for an SEV guest can not be supported by the
> hypervisor, unroll the instructions for CC_ATTR_GUEST_UNROLL_STRING_IO
> enabled kernels.
>
> This issue appears when kernels are launched in recent libvirt-managed
> SEV virtual machines, because libvirt started to add a tpm-crb device
> to the guest by default.
>
> The kernel driver for tpm-crb uses memcpy_to/from_io() functions to
> access MMIO memory, resulting in a page-fault injected by KVM and
> crashing the kernel at boot.
>
> Cc: [email protected] #4.15+
> Fixes: d8aa7eea78a1 ('x86/mm: Add Secure Encrypted Virtualization (SEV) support')
> Reviewed-by: Tom Lendacky <[email protected]>
> Signed-off-by: Joerg Roedel <[email protected]>
> ---
> Changes v2->v3:
> - Fix sparse warnings introduced by v2
>
> arch/x86/lib/iomem.c | 65 ++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 57 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/lib/iomem.c b/arch/x86/lib/iomem.c
> index df50451d94ef..3e2f33fc33de 100644
> --- a/arch/x86/lib/iomem.c
> +++ b/arch/x86/lib/iomem.c
> @@ -22,7 +22,7 @@ static __always_inline void rep_movs(void *to, const void *from, size_t n)
> : "memory");
> }
>
> -void memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
> +static void string_memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
> {
> if (unlikely(!n))
> return;
> @@ -38,9 +38,8 @@ void memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
> }
> rep_movs(to, (const void *)from, n);
> }
> -EXPORT_SYMBOL(memcpy_fromio);
>
> -void memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
> +static void string_memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
> {
> if (unlikely(!n))
> return;
> @@ -56,14 +55,64 @@ void memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
> }
> rep_movs((void *)to, (const void *) from, n);
> }
> +
> +static void unrolled_memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
> +{
> + const volatile char __iomem *in = from;
> + char *out = to;
> + int i;
> +
> + for (i = 0; i < n; ++i)
> + out[i] = readb(&in[i]);
> +}
Wait a minute....
Aren't these functions supposed to be doing 'memory' copies?
In which case they need to be using 64bit IO accesses where
appropriate - otherwise the performance is horrid.
I thought the x86 memcpy_to/from_io() had been changed to
always use a software loop rather than using whatever memcpy()
ended up using.
In particular the 'rep movsb' ERMS (EMRS?) copy that is fast
(on some cpu) for memory-memory copies is always a byte copy
on uncached locations typical for io addresses.
PIO reads from PCIe can be spectacularly slow.
You really do want to use the largest register available.
David
-
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