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Message-ID: <27e31afd-2f8e-4f2e-92e3-92e52b956751@intel.com>
Date: Tue, 25 Feb 2025 13:56:41 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Adrian Hunter <adrian.hunter@...el.com>, pbonzini@...hat.com,
seanjc@...gle.com
Cc: kvm@...r.kernel.org, rick.p.edgecombe@...el.com, kai.huang@...el.com,
reinette.chatre@...el.com, tony.lindgren@...ux.intel.com,
binbin.wu@...ux.intel.com, dmatlack@...gle.com, isaku.yamahata@...el.com,
nik.borisov@...e.com, linux-kernel@...r.kernel.org, yan.y.zhao@...el.com,
chao.gao@...el.com, weijiang.yang@...el.com
Subject: Re: [PATCH V2 02/12] KVM: x86: Allow the use of
kvm_load_host_xsave_state() with guest_state_protected
On 2/24/2025 7:38 PM, Adrian Hunter wrote:
> On 20/02/25 12:50, Xiaoyao Li wrote:
>> On 1/29/2025 5:58 PM, Adrian Hunter wrote:
>>> From: Sean Christopherson <seanjc@...gle.com>
>>>
>>> Allow the use of kvm_load_host_xsave_state() with
>>> vcpu->arch.guest_state_protected == true. This will allow TDX to reuse
>>> kvm_load_host_xsave_state() instead of creating its own version.
>>>
>>> For consistency, amend kvm_load_guest_xsave_state() also.
>>>
>>> Ensure that guest state that kvm_load_host_xsave_state() depends upon,
>>> such as MSR_IA32_XSS, cannot be changed by user space, if
>>> guest_state_protected.
>>>
>>> [Adrian: wrote commit message]
>>>
>>> Link: https://lore.kernel.org/r/Z2GiQS_RmYeHU09L@google.com
>>> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>>> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
>>> ---
>>> TD vcpu enter/exit v2:
>>> - New patch
>>> ---
>>> arch/x86/kvm/svm/svm.c | 7 +++++--
>>> arch/x86/kvm/x86.c | 18 +++++++++++-------
>>> 2 files changed, 16 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>>> index 7640a84e554a..b4bcfe15ad5e 100644
>>> --- a/arch/x86/kvm/svm/svm.c
>>> +++ b/arch/x86/kvm/svm/svm.c
>>> @@ -4253,7 +4253,9 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu,
>>> svm_set_dr6(svm, DR6_ACTIVE_LOW);
>>> clgi();
>>> - kvm_load_guest_xsave_state(vcpu);
>>> +
>>> + if (!vcpu->arch.guest_state_protected)
>>> + kvm_load_guest_xsave_state(vcpu);
>>> kvm_wait_lapic_expire(vcpu);
>>> @@ -4282,7 +4284,8 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu,
>>> if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
>>> kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
>>> - kvm_load_host_xsave_state(vcpu);
>>> + if (!vcpu->arch.guest_state_protected)
>>> + kvm_load_host_xsave_state(vcpu);
>>> stgi();
>>> /* Any pending NMI will happen here */
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index bbb6b7f40b3a..5cf9f023fd4b 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -1169,11 +1169,9 @@ EXPORT_SYMBOL_GPL(kvm_lmsw);
>>> void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
>>> {
>>> - if (vcpu->arch.guest_state_protected)
>>> - return;
>>> + WARN_ON_ONCE(vcpu->arch.guest_state_protected);
>>> if (kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE)) {
>>> -
>>> if (vcpu->arch.xcr0 != kvm_host.xcr0)
>>> xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
>>> @@ -1192,13 +1190,11 @@ EXPORT_SYMBOL_GPL(kvm_load_guest_xsave_state);
>>> void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
>>> {
>>> - if (vcpu->arch.guest_state_protected)
>>> - return;
>>> -
>>> if (cpu_feature_enabled(X86_FEATURE_PKU) &&
>>> ((vcpu->arch.xcr0 & XFEATURE_MASK_PKRU) ||
>>> kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE))) {
>>> - vcpu->arch.pkru = rdpkru();
>>> + if (!vcpu->arch.guest_state_protected)
>>> + vcpu->arch.pkru = rdpkru();
>>
>> this needs justification.
>
> It was proposed by Sean here:
>
> https://lore.kernel.org/all/Z2WZ091z8GmGjSbC@google.com/
>
> which is part of the email thread referenced by the "Link:" tag above
IMHO, this change needs to be put in patch 07, which is the better place
to justify it.
>>
>>> if (vcpu->arch.pkru != vcpu->arch.host_pkru)
>>> wrpkru(vcpu->arch.host_pkru);
>>> }
>>
>>
>>> @@ -3916,6 +3912,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>> if (!msr_info->host_initiated &&
>>> !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
>>> return 1;
>>> +
>>> + if (vcpu->arch.guest_state_protected)
>>> + return 1;
>>> +
>>
>> this and below change need to be a separate patch. So that we can discuss independently.
>>
>> I see no reason to make MSR_IA32_XSS special than other MSRs. When guest_state_protected, most of the MSRs that aren't emulated by KVM are inaccessible by KVM.
>
> Yes, TDX will block access to MSR_IA32_XSS anyway because
> tdx_has_emulated_msr() will return false for MSR_IA32_XSS.
>
> However kvm_load_host_xsave_state() is not TDX-specific code and it
> relies upon vcpu->arch.ia32_xss, so there is reason to block
> access to it when vcpu->arch.guest_state_protected is true.
It is TDX specific logic that TDX requires vcpu->arch.ia32_xss unchanged
since TDX is going to utilize kvm_load_host_xsave_state() to restore
host xsave state and relies on vcpu->arch.ia32_xss to be always the
value of XFAM & XSS_MASK.
So please put this change into the TDX specific patch with the clear
justfication.
>>
>>> /*
>>> * KVM supports exposing PT to the guest, but does not support
>>> * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
>>> @@ -4375,6 +4375,10 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>> if (!msr_info->host_initiated &&
>>> !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
>>> return 1;
>>> +
>>> + if (vcpu->arch.guest_state_protected)
>>> + return 1;
>>> +
>>> msr_info->data = vcpu->arch.ia32_xss;
>>> break;
>>> case MSR_K7_CLK_CTL:
>>
>
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