linux-kernel - Re: [PATCH v3 05/10] KVM: TDX: restore host xsave state when exit from the guest TD

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Message-ID: <CABgObfb50fyDXnQawQQregR6UzH5mZukjX9iADtcrv1VemrVXg@mail.gmail.com>
Date: Thu, 13 Mar 2025 19:17:53 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, adrian.hunter@...el.com, 
	seanjc@...gle.com, rick.p.edgecombe@...el.com, 
	Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH v3 05/10] KVM: TDX: restore host xsave state when exit
 from the guest TD

On Thu, Mar 13, 2025 at 4:17 AM Xiaoyao Li <xiaoyao.li@...el.com> wrote:
>
> On 3/12/2025 7:36 PM, Paolo Bonzini wrote:
> > On Mon, Mar 10, 2025 at 8:24 AM Xiaoyao Li <xiaoyao.li@...el.com> wrote:
> >>
> >> On 3/8/2025 5:20 AM, Paolo Bonzini wrote:
> >>> From: Isaku Yamahata <isaku.yamahata@...el.com>
> >>>
> >>> On exiting from the guest TD, xsave state is clobbered; restore it.
> >>
> >> I prefer the implementation as this patch, which is straightforward.
> >> (I would be much better if the changelog can describe more)
> >
> > Ok:
> >
> > Do not use kvm_load_host_xsave_state(), as it relies on vcpu->arch
> > to find out whether other KVM_RUN code has loaded guest state into
> > XCR0/PKRU/XSS or not.  In the case of TDX, the exit values are known
> > independent of the guest CR0 and CR4, and in fact the latter are not
> > available.
>
> In fact, I expected some description of how xsave state is clobbered and
> what value of them after TD exit.
>
>    After return from TDH.VP.ENTER, XCR0 is set to TD's user-mode feature
>    bits of XFAM and MSR_IA32_XSS is set to TD's supervisor-mode feature
>    bits of XFAM. PKRU keeps unchanged if the TD is not exposed with PKU
>    in XFAM or PKRU is set to 0 when XFAM.PKE(bit 9) is 1.

Ah, I didn't include that because it's just information from the TDX
module documentation.

Paolo