linux-kernel - Re: [PATCH v5 1/4] KVM: TDX: Explicitly set user-return MSRs that *may* be clobbered by the TDX-Module

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Message-ID: <aQo-KhJ9nb0MMAy4@google.com>
Date: Tue, 4 Nov 2025 09:55:54 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Xiaoyao Li <xiaoyao.li@...el.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	"Kirill A. Shutemov" <kas@...nel.org>, kvm@...r.kernel.org, x86@...nel.org, 
	linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Hou Wenlong <houwenlong.hwl@...group.com>
Subject: Re: [PATCH v5 1/4] KVM: TDX: Explicitly set user-return MSRs that
 *may* be clobbered by the TDX-Module

On Tue, Nov 04, 2025, Yan Zhao wrote:
> On Tue, Nov 04, 2025 at 04:40:44PM +0800, Xiaoyao Li wrote:
> > On 11/4/2025 3:06 PM, Yan Zhao wrote:
> > > Another nit:
> > > Remove the tdx_user_return_msr_update_cache() in the comment of __tdx_bringup().
> > > 
> > > Or could we just invoke tdx_user_return_msr_update_cache() in
> > > tdx_prepare_switch_to_guest()?
> > 
> > No. It lacks the WRMSR operation to update the hardware value, which is the
> > key of this patch.
> As [1], I don't think the WRMSR operation to update the hardware value is
> necessary. The value will be updated to guest value soon any way if
> tdh_vp_enter() succeeds, or the hardware value remains to be the host value or
> the default value.

As explained in the original thread:  

 : > If the MSR's do not get clobbered, does it matter whether or not they get
 : > restored.
 : 
 : It matters because KVM needs to know the actual value in hardware.  If KVM thinks
 : an MSR is 'X', but it's actually 'Y', then KVM could fail to write the correct
 : value into hardware when returning to userspace and/or when running a different
 : vCPU.

I.e. updating the cache effectively corrupts state if the TDX-Module doesn't
clobber MSRs as expected, i.e. if the current value is preserved in hardware.

> But I think invoking tdx_user_return_msr_update_cache() in
> tdx_prepare_switch_to_guest() is better than in
> tdx_prepare_switch_to_host().
> 
> [1] https://lore.kernel.org/kvm/aQhJol0CvT6bNCJQ@yzhao56-desk.sh.intel.com/
>