linux-kernel - Re: [PATCH 3/9] KVM: x86: SVM: Pass through shadow stack MSRs

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Message-ID: <c65817b0-7fa6-7c0b-6423-5f33062c9665@amd.com>
Date:   Wed, 18 Oct 2023 16:57:15 +0530
From:   "Nikunj A. Dadhania" <nikunj@....com>
To:     John Allen <john.allen@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, weijiang.yang@...el.com,
        rick.p.edgecombe@...el.com, seanjc@...gle.com, x86@...nel.org,
        thomas.lendacky@....com, bp@...en8.de
Subject: Re: [PATCH 3/9] KVM: x86: SVM: Pass through shadow stack MSRs

On 10/17/2023 11:47 PM, John Allen wrote:
> On Thu, Oct 12, 2023 at 02:31:19PM +0530, Nikunj A. Dadhania wrote:
>> On 10/11/2023 1:32 AM, John Allen wrote:
>>> If kvm supports shadow stack, pass through shadow stack MSRs to improve
>>> guest performance.
>>>
>>> Signed-off-by: John Allen <john.allen@....com>
>>> ---
>>>  arch/x86/kvm/svm/svm.c | 26 ++++++++++++++++++++++++++
>>>  arch/x86/kvm/svm/svm.h |  2 +-
>>>  2 files changed, 27 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>>> index e435e4fbadda..984e89d7a734 100644
>>> --- a/arch/x86/kvm/svm/svm.c
>>> +++ b/arch/x86/kvm/svm/svm.c
>>> @@ -139,6 +139,13 @@ static const struct svm_direct_access_msrs {
>>>  	{ .index = X2APIC_MSR(APIC_TMICT),		.always = false },
>>>  	{ .index = X2APIC_MSR(APIC_TMCCT),		.always = false },
>>>  	{ .index = X2APIC_MSR(APIC_TDCR),		.always = false },
>>> +	{ .index = MSR_IA32_U_CET,                      .always = false },
>>> +	{ .index = MSR_IA32_S_CET,                      .always = false },
>>> +	{ .index = MSR_IA32_INT_SSP_TAB,                .always = false },
>>> +	{ .index = MSR_IA32_PL0_SSP,                    .always = false },
>>> +	{ .index = MSR_IA32_PL1_SSP,                    .always = false },
>>> +	{ .index = MSR_IA32_PL2_SSP,                    .always = false },
>>> +	{ .index = MSR_IA32_PL3_SSP,                    .always = false },
>>
>> First three MSRs are emulated in the patch 1, any specific reason for skipping MSR_IA32_PL[0-3]_SSP ?
> 
> I'm not sure what you mean. 

MSR_IA32_U_CET, MSR_IA32_S_CET and MSR_IA32_INT_SSP_TAB are selectively emulated and there is no good explanation why MSR_IA32_PL[0-3]_SSP do not need emulation. Moreover, MSR interception is initially set (i.e. always=false) for MSR_IA32_PL[0-3]_SSP.

> The PLx_SSP MSRS should be getting passed
> through here unless I'm misunderstanding something.

In that case, intercept should be cleared from the very beginning.

+	{ .index = MSR_IA32_PL0_SSP,                    .always = true },
+	{ .index = MSR_IA32_PL1_SSP,                    .always = true },
+	{ .index = MSR_IA32_PL2_SSP,                    .always = true },
+	{ .index = MSR_IA32_PL3_SSP,                    .always = true },

Regards
Nikunj