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Re: [PATCH v2 9/9] x86/cpuid: Enable MSR_SPEC_CTRL in SVM guests by default
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 31 Jan 2022 11:39:12 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 31 Jan 2022 10:39:21 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 28.01.2022 14:29, Andrew Cooper wrote:
> With all other pieces in place, MSR_SPEC_CTRL is fully working for HVM guests.
>
> Update the CPUID derivation logic (both PV and HVM to avoid losing subtle
> changes), drop the MSR intercept, and explicitly enable the CPUID bits for HVM
> guests.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Oneremark:
> --- a/xen/arch/x86/hvm/svm/svm.c
> +++ b/xen/arch/x86/hvm/svm/svm.c
> @@ -606,6 +606,10 @@ static void svm_cpuid_policy_changed(struct vcpu *v)
>
> vmcb_set_exception_intercepts(vmcb, bitmap);
>
> + /* Give access to MSR_SPEC_CTRL if the guest has been told about it. */
> + svm_intercept_msr(v, MSR_SPEC_CTRL,
> + cp->extd.ibrs ? MSR_INTERCEPT_NONE : MSR_INTERCEPT_RW);
Technically I suppose the intercept would also be unneeded if the MSR
doesn't exist at all, as then the CPU would raise #GP(0) for any guest
attempt to access it.
Jan
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