2024-01-15 17:58:32

by Saurabh Singh Sengar

Subject: [PATCH v2] x86/hyperv: Allow 15-bit APIC IDs for VTL platforms

The current method for signaling the compatibility of a Hyper-V host
with MSIs featuring 15-bit APIC IDs relies on a synthetic cpuid leaf.
However, for higher VTLs, this leaf is not reported, due to the absence
of an IO-APIC.

As an alternative, assume that when running at a high VTL, the host
supports 15-bit APIC IDs. This assumption is safe, as Hyper-V does not
employ any architectural MSIs at higher VTLs

This unblocks startup of VTL2 environments with more than 256 CPUs.

Signed-off-by: Saurabh Sengar <[email protected]>
---
[V2]
- Modify commit message

arch/x86/hyperv/hv_vtl.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
index 96e6c51..cf1b78c 100644
--- a/arch/x86/hyperv/hv_vtl.c
+++ b/arch/x86/hyperv/hv_vtl.c
@@ -16,6 +16,11 @@
extern struct boot_params boot_params;
static struct real_mode_header hv_vtl_real_mode_header;

+static bool __init hv_vtl_msi_ext_dest_id(void)
+{
+ return true;
+}
+
void __init hv_vtl_init_platform(void)
{
pr_info("Linux runs in Hyper-V Virtual Trust Level\n");
@@ -38,6 +43,8 @@ void __init hv_vtl_init_platform(void)
x86_platform.legacy.warm_reset = 0;
x86_platform.legacy.reserve_bios_regions = 0;
x86_platform.legacy.devices.pnpbios = 0;
+
+ x86_init.hyper.msi_ext_dest_id = hv_vtl_msi_ext_dest_id;
}

static inline u64 hv_vtl_system_desc_base(struct ldttss_desc *desc)
--
1.8.3.1

2024-01-16 15:26:27

by Michael Kelley

Subject: RE: [PATCH v2] x86/hyperv: Allow 15-bit APIC IDs for VTL platforms

From: Saurabh Sengar <[email protected]> Sent: Monday, January 15, 2024 9:58 AM
>
> The current method for signaling the compatibility of a Hyper-V host
> with MSIs featuring 15-bit APIC IDs relies on a synthetic cpuid leaf.
> However, for higher VTLs, this leaf is not reported, due to the absence
> of an IO-APIC.
>
> As an alternative, assume that when running at a high VTL, the host
> supports 15-bit APIC IDs. This assumption is safe, as Hyper-V does not
> employ any architectural MSIs at higher VTLs
>
> This unblocks startup of VTL2 environments with more than 256 CPUs.
>
> Signed-off-by: Saurabh Sengar <[email protected]>
> ---
> [V2]
> - Modify commit message
>
> arch/x86/hyperv/hv_vtl.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> index 96e6c51..cf1b78c 100644
> --- a/arch/x86/hyperv/hv_vtl.c
> +++ b/arch/x86/hyperv/hv_vtl.c
> @@ -16,6 +16,11 @@
> extern struct boot_params boot_params;
> static struct real_mode_header hv_vtl_real_mode_header;
>
> +static bool __init hv_vtl_msi_ext_dest_id(void)
> +{
> + return true;
> +}
> +
> void __init hv_vtl_init_platform(void)
> {
> pr_info("Linux runs in Hyper-V Virtual Trust Level\n");
> @@ -38,6 +43,8 @@ void __init hv_vtl_init_platform(void)
> x86_platform.legacy.warm_reset = 0;
> x86_platform.legacy.reserve_bios_regions = 0;
> x86_platform.legacy.devices.pnpbios = 0;
> +
> + x86_init.hyper.msi_ext_dest_id = hv_vtl_msi_ext_dest_id;
> }
>
> static inline u64 hv_vtl_system_desc_base(struct ldttss_desc *desc)
> --
> 1.8.3.1
>

Reviewed-by: Michael Kelley <[email protected]>

2024-03-01 09:37:18

by Wei Liu

Subject: Re: [PATCH v2] x86/hyperv: Allow 15-bit APIC IDs for VTL platforms

On Tue, Jan 16, 2024 at 03:25:47PM +0000, Michael Kelley wrote:
> From: Saurabh Sengar <[email protected]> Sent: Monday, January 15, 2024 9:58 AM
> >
> > The current method for signaling the compatibility of a Hyper-V host
> > with MSIs featuring 15-bit APIC IDs relies on a synthetic cpuid leaf.
> > However, for higher VTLs, this leaf is not reported, due to the absence
> > of an IO-APIC.
> >
> > As an alternative, assume that when running at a high VTL, the host
> > supports 15-bit APIC IDs. This assumption is safe, as Hyper-V does not
> > employ any architectural MSIs at higher VTLs
> >
> > This unblocks startup of VTL2 environments with more than 256 CPUs.
> >
> > Signed-off-by: Saurabh Sengar <[email protected]>
[...]
>
> Reviewed-by: Michael Kelley <[email protected]>

Applied. Thanks.