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: <aQnH3EmN97cAKDEO@yzhao56-desk.sh.intel.com>
Date: Tue, 4 Nov 2025 17:31:08 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
CC: Sean Christopherson <seanjc@...gle.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 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.

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/