linux-kernel - Re: [PATCH v3 24/25] KVM: TDX: Guard VM state transitions with "all" the locks

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Message-ID: <aQAQLzxyGFooGtNV@google.com>
Date: Mon, 27 Oct 2025 17:37:03 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
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Subject: Re: [PATCH v3 24/25] KVM: TDX: Guard VM state transitions with "all"
 the locks

On Tue, Oct 28, 2025, Kai Huang wrote:
> On Thu, 2025-10-16 at 17:32 -0700, Sean Christopherson wrote:
> > @@ -2781,8 +2827,6 @@ int tdx_vm_ioctl(struct kvm *kvm, void __user *argp)
> >  	if (r)
> >  		return r;
> >  
> > -	guard(mutex)(&kvm->lock);
> > -
> >  	switch (tdx_cmd.id) {
> >  	case KVM_TDX_CAPABILITIES:
> >  		r = tdx_get_capabilities(&tdx_cmd);
> 
> IIRC, this patch removes grabbing the kvm->lock in tdx_vm_ioctl() but only
> adds the "big hammer" to tdx_td_init() and tdx_td_finalize(), so the
> tdx_get_capabilities() lost holding the kvm->lock.

Ooh, yeah, nice catch, that is indeed silly and unnecessary churn.

> As replied earlier, I think we can just hold the "big hammer" in
> tdx_vm_ioctl()?

Actually, I think we can have our cake and eat it too.  With this slotted in as
a prep patch, the big hammer can land directly in tdx_vm_ioctl(), without any
change in functionality for KVM_TDX_CAPABILITIES.

--
From: Sean Christopherson <seanjc@...gle.com>
Date: Mon, 27 Oct 2025 17:32:34 -0700
Subject: [PATCH] KVM: TDX: Don't copy "cmd" back to userspace for
 KVM_TDX_CAPABILITIES

Don't copy the kvm_tdx_cmd structure back to userspace when handling
KVM_TDX_CAPABILITIES, as tdx_get_capabilities() doesn't modify hw_error or
any other fields.

Opportunistically hoist the call to tdx_get_capabilities() outside of the
kvm->lock critical section, as getting the capabilities doesn't touch the
VM in any way, e.g. doesn't even take @kvm.

Suggested-by: Kai Huang <kai.huang@...el.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/tdx.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 1642da9c1fa9..43c0c3f6a8c0 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -2807,12 +2807,12 @@ int tdx_vm_ioctl(struct kvm *kvm, void __user *argp)
 	if (r)
 		return r;
 
+	if (tdx_cmd.id == KVM_TDX_CAPABILITIES)
+		return tdx_get_capabilities(&tdx_cmd);
+
 	guard(mutex)(&kvm->lock);
 
 	switch (tdx_cmd.id) {
-	case KVM_TDX_CAPABILITIES:
-		r = tdx_get_capabilities(&tdx_cmd);
-		break;
 	case KVM_TDX_INIT_VM:
 		r = tdx_td_init(kvm, &tdx_cmd);
 		break;

base-commit: 672537233b8da2c29dca7154bf3a3211af7f6128
--