linux-kernel - Re: [PATCH] KVM: x86: Drop "cache" from user return MSR setter that skips WRMSR

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Message-ID: <aNwGjIoNRGZL3_Qr@google.com>
Date: Tue, 30 Sep 2025 09:34:20 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH] KVM: x86: Drop "cache" from user return MSR setter that
 skips WRMSR

On Tue, Sep 30, 2025, Sean Christopherson wrote:
> On Tue, Sep 30, 2025, Yan Zhao wrote:
> > On Tue, Sep 30, 2025 at 08:22:41PM +0800, Yan Zhao wrote:
> > > On Fri, Sep 19, 2025 at 02:42:59PM -0700, Sean Christopherson wrote:
> > > > Rename kvm_user_return_msr_update_cache() to __kvm_set_user_return_msr()
> > > > and use the helper kvm_set_user_return_msr() to make it obvious that the
> > > > double-underscores version is doing a subset of the work of the "full"
> > > > setter.
> > > > 
> > > > While the function does indeed update a cache, the nomenclature becomes
> > > > slightly misleading when adding a getter[1], as the current value isn't
> > > > _just_ the cached value, it's also the value that's currently loaded in
> > > > hardware.
> > > Nit:
> > > 
> > > For TDX, "it's also the value that's currently loaded in hardware" is not true.
> > since tdx module invokes wrmsr()s before each exit to VMM, while KVM only
> > invokes __kvm_set_user_return_msr() in tdx_vcpu_put().
> 
> No?  kvm_user_return_msr_update_cache() is passed the value that's currently
> loaded in hardware, by way of the TDX-Module zeroing some MSRs on TD-Exit.
> 
> Ah, I suspect you're calling out that the cache can be stale.  Maybe this?
> 
>   While the function does indeed update a cache, the nomenclature becomes
>   slightly misleading when adding a getter[1], as the current value isn't
>   _just_ the cached value, it's also the value that's currently loaded in
>   hardware (ignoring that the cache holds stale data until the vCPU is put,
>   i.e. until KVM prepares to switch back to the host).
> 
> Actually, that's a bug waiting to happen when the getter comes along.  Rather
> than document the potential pitfall, what about adding a prep patch to mimize
> the window?  Then _this_ patch shouldn't need the caveat about the cache being
> stale.

Ha!  It's technically a bug fix.  Because a forced shutdown will invoke
kvm_shutdown() without waiting for tasks to exit, and so the on_each_cpu() calls
to kvm_disable_virtualization_cpu() can call kvm_on_user_return() and thus
consume a stale values->curr.