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Message-ID: <7de6b755-3cf9-4d1c-11e8-3458e6764545@redhat.com>
Date: Mon, 13 Dec 2021 10:01:23 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Liu, Jing2" <jing2.liu@...el.com>,
"Zhong, Yang" <yang.zhong@...el.com>,
"x86@...nel.org" <x86@...nel.org>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"mingo@...hat.com" <mingo@...hat.com>,
"bp@...en8.de" <bp@...en8.de>,
"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Cc: "Christopherson,, Sean" <seanjc@...gle.com>,
"Nakajima, Jun" <jun.nakajima@...el.com>,
"Tian, Kevin" <kevin.tian@...el.com>,
"jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
"Wang, Wei W" <wei.w.wang@...el.com>,
"Zeng, Guang" <guang.zeng@...el.com>
Subject: Re: [PATCH 10/19] kvm: x86: Emulate WRMSR of guest IA32_XFD
On 12/13/21 08:51, Liu, Jing2 wrote:
> On 12/11/2021 12:02 AM, Paolo Bonzini wrote:
>>
>> Also:
>>
>> On 12/8/21 01:03, Yang Zhong wrote:
>>>
>>> + if (!guest_cpuid_has(vcpu, X86_FEATURE_XFD))
>>> + return 1;
>>
>> This should allow msr->host_initiated always (even if XFD is not part of
>> CPUID).
> Thanks Paolo.
>
> msr->host_initiated handling would be added in next version.
>
> I'd like to ask why always allow msr->host_initiated even if XFD is not part of
> CPUID, although guest doesn't care that MSR? We found some MSRs
> (e.g. MSR_AMD64_OSVW_STATUS and MSR_AMD64_OSVW_ID_LENGTH )
> are specially handled so would like to know the consideration of allowing
> msr->host_initiated.
>
> if (!msr_info->host_initiated && !guest_cpuid_has(vcpu, X86_FEATURE_XFD))
> return 1;
Because it's simpler if userspace can just take the entire list from
KVM_GET_MSR_INDEX_LIST and pass it to KVM_GET/SET_MSR. See for example
vcpu_save_state and vcpu_load_state in
tools/testing/selftests/kvm/lib/x86_64/processor.c.
>> However, if XFD is nonzero and kvm_check_guest_realloc_fpstate
>> returns true, then it should return 1.
>
> If XFD is nonzero, kvm_check_guest_realloc_fpstate() won't return true. So
> may not need this check here?
It can't for now, because there's a single dynamic feature, but here:
+ if ((xfd & xcr0) != xcr0) {
+ u64 request = (xcr0 ^ xfd) & xcr0;
+ struct fpu_guest *guest_fpu = &vcpu->arch.guest_fpu;
+
+ /*
+ * If requested features haven't been enabled, update
+ * the request bitmap and tell the caller to request
+ * dynamic buffer reallocation.
+ */
+ if ((guest_fpu->user_xfeatures & request) != request) {
+ vcpu->arch.guest_fpu.realloc_request = request;
+ return true;
+ }
+ }
it is certainly possible to return true with nonzero XFD.
Paolo
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