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Re: [PATCH v4 2/3] amd/msr: allow passthrough of VIRT_SPEC_CTRL for HVM guests
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 29 Apr 2022 12:35:45 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Fri, 29 Apr 2022 10:35:52 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27.04.2022 12:47, Roger Pau Monne wrote:
> --- a/xen/arch/x86/cpuid.c
> +++ b/xen/arch/x86/cpuid.c
> @@ -541,6 +541,10 @@ static void __init calculate_hvm_max_policy(void)
> raw_cpuid_policy.basic.sep )
> __set_bit(X86_FEATURE_SEP, hvm_featureset);
>
> + if ( !boot_cpu_has(X86_FEATURE_VIRT_SC_MSR_HVM) )
> + /* Clear VIRT_SSBD if VIRT_SPEC_CTRL is not exposed to guests. */
> + __clear_bit(X86_FEATURE_VIRT_SSBD, hvm_featureset);
> +
> /*
> * If Xen isn't virtualising MSR_SPEC_CTRL for HVM guests (functional
> * availability, or admin choice), hide the feature.
> @@ -597,6 +601,13 @@ static void __init calculate_hvm_def_policy(void)
> guest_common_feature_adjustments(hvm_featureset);
> guest_common_default_feature_adjustments(hvm_featureset);
>
> + /*
> + * Only expose VIRT_SSBD if AMD_SSBD is not available, and thus
> + * VIRT_SC_MSR_HVM is set.
> + */
> + if ( boot_cpu_has(X86_FEATURE_VIRT_SC_MSR_HVM) )
> + __set_bit(X86_FEATURE_VIRT_SSBD, hvm_featureset);
The earlier patch sets the bit in "max" when SC_MSR_HVM && AMD_SSBD.
This patch doesn't set the bit in "max" at all (it only clears it in
one case as per the earlier hunk), thus leading to "def" holding a
set bit which supposedly cannot be set. Irrespective of the feature's
'!' annotation I think we'd better not violate "max" >= "def".
Everything else in this patch looks good to me.
Jan
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