Re: [PATCH v10 00/28] KVM: Enable FRED support with KVM VMX

[Sean Christopherson]

On Sat, Oct 24, 2026, H. Peter Anvin wrote:
> On 2026-09-14 23:03, Xin Li wrote:
> > 
> >> On Sep 11, 2026, at 2:36 PM, Sohil Mehta <sohil.mehta@xxxxxxxxx> wrote:
> >>
> >> I mainly rebased the patches to the latest kvm-x86/next branch and fixed
> >> a handful of issues reported during internal testing and basic AI
> >> reviews. I've avoided making any major code changes or commit message
> >> updates as this is the first iteration that I am posting. 
> > 
> > Thanks a lot for taking care of the patch series.
> > 
> >>
> >> Going forward, based on the feedback, I'll rework the patches as needed.
> >> I've listed a few opens below that need additional discussion or
> >> clarification. I'm relatively new to the KVM subsystem, so please feel
> >> free to provide suggestions on the series structure, code organization
> >> as well as commit log clarity to smoothen the review process.
> > 
> > I’ve been thinking about an optimization: because FRED drops most of the old    
> > syscall MSRs (IA32_LSTAR/IA32_CSTAR/IA32_SFMASK), we should be able to omit     
> > them from KVM's uret MSR tracking entirely as if host enables FRED (no matter   
> > whether a guest uses FRED).
> > 
> Yes, although they will need to be handled properly between VCPU switches, of
> course.

And we'd want to keep the optimizations to elide writes when vCPUs are using the
same value(s).  So, compile tested only...

From: Sean Christopherson <seanjc@xxxxxxxxxx>
Date: Mon, 26 Oct 2026 07:37:03 -0700
Subject: [PATCH] KVM: x86: Don't bother loading host values for SYSCALL MSRs
 when FRED is in use

Elide the WRMSRs for the SYSCALL MSRs when returning to userspace if FRED
is in use, in which case said MSRs are never actually used in Linux.

Note, KVM never saves/restores MSR_CSTAR via the user-return MSR mechanism,
as the MSR is unused by Intel/VMX hardware, and SVM unconditionally context
switches the MSR via a dedicated VMCB field (along with all other SYSCALL
MSRs).  Include MSR_CSTAR to provide a contiguous range of values in the
case statements, e.g. so that the compiler can optimize the case statements
into a simple bounds checks.

Suggested-by: Xin Li (Intel) <xin@xxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/msrs.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/msrs.c b/arch/x86/kvm/msrs.c
index e91650af44c2..ef3596d8c780 100644
--- a/arch/x86/kvm/msrs.c
+++ b/arch/x86/kvm/msrs.c
@@ -65,6 +65,23 @@ void kvm_destroy_user_return_msrs(void)
 	kvm_nr_uret_msrs = 0;
 }
 
+static bool kvm_is_msr_used_in_host(u32 msr)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_FRED))
+		return true;
+
+	switch (msr) {
+	case MSR_STAR:
+	case MSR_LSTAR:
+	case MSR_CSTAR:
+	case MSR_SYSCALL_MASK:
+		return false;
+	default:
+		break;
+	}
+	return true;
+}
+
 static void kvm_on_user_return(struct user_return_notifier *urn)
 {
 	unsigned slot;
@@ -77,7 +94,8 @@ static void kvm_on_user_return(struct user_return_notifier *urn)
 
 	for (slot = 0; slot < kvm_nr_uret_msrs; ++slot) {
 		values = &msrs->values[slot];
-		if (values->host != values->curr) {
+		if (kvm_is_msr_used_in_host(kvm_uret_msrs_list[slot]) &&
+		    values->host != values->curr) {
 			wrmsrq(kvm_uret_msrs_list[slot], values->host);
 			values->curr = values->host;
 		}

base-commit: 7e60162b16f9da0d2fa83da5c1723b6853a0ac1d
--