linux-kernel - Re: [PATCH 4/7] KVM: TDX: restore host xsave state when exit from the guest TD

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Message-ID: <Z4cdShy8Cuitd44I@google.com>
Date: Tue, 14 Jan 2025 18:28:26 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: Chao Gao <chao.gao@...el.com>, pbonzini@...hat.com, kvm@...r.kernel.org, 
	dave.hansen@...ux.intel.com, rick.p.edgecombe@...el.com, kai.huang@...el.com, 
	reinette.chatre@...el.com, xiaoyao.li@...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, 
	x86@...nel.org, yan.y.zhao@...el.com, weijiang.yang@...el.com
Subject: Re: [PATCH 4/7] KVM: TDX: restore host xsave state when exit from the
 guest TD

On Tue, Jan 14, 2025, Adrian Hunter wrote:
> On 10/01/25 19:30, Sean Christopherson wrote:
> >> Currently the KVM selftests expect to be able to set XCR0:
> >>
> >>     td_vcpu_add()
> >> 	vm_vcpu_add()
> >> 	    vm_arch_vcpu_add()
> >> 		vcpu_init_xcrs()
> >> 		    vcpu_xcrs_set()
> >> 			vcpu_ioctl(KVM_SET_XCRS)
> >> 			    __TEST_ASSERT_VM_VCPU_IOCTL(!ret)
> >>
> >> Seems like vm->arch.has_protected_state is needed for KVM selftests?
> > 
> > I doubt it's truly needed, my guess (without looking at the code) is that selftests
> > are fudging around the fact that KVM doesn't stuff arch.xcr0.
> 
> Here is when it was added:
> 
> commit 8b14c4d85d031f7700fa4e042aebf99d933971f0
> Author: Sean Christopherson <seanjc@...gle.com>
> Date:   Thu Oct 3 16:43:31 2024 -0700
> 
>     KVM: selftests: Configure XCR0 to max supported value by default
>     
>     To play nice with compilers generating AVX instructions, set CR4.OSXSAVE
>     and configure XCR0 by default when creating selftests vCPUs.  Some distros
>     have switched gcc to '-march=x86-64-v3' by default, and while it's hard to
>     find a CPU which doesn't support AVX today, many KVM selftests fail with

Gah, sorry.  I misread the callstack the first time around and didn't realize it
was the common code that was writing XCR0.

> Is below OK to avoid it?

Skipping the ioctls to set XCRs and SREGS is definitely ok.  I'll hold off on
providing concrete feedback until I review the TDX selftests in its entirety,
as I'm skeptical of having td_vcpu_add() wrap vm_arch_vcpu_add() instead of the
other way around, but I don't want to cause a bunch of noise by reacting to a
sliver of the code.