linux-kernel - Re: [PATCH v5 1/9] KVM: VMX: Use on-stack copy of @flags in __vmx_vcpu

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Message-ID: <DE8F7MGETJP2.K6A8JKD8LV4A@google.com>
Date: Fri, 14 Nov 2025 12:36:01 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>
Cc: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Brendan Jackman <jackmanb@...gle.com>
Subject: Re: [PATCH v5 1/9] KVM: VMX: Use on-stack copy of @flags in __vmx_vcpu_run()

On Thu Nov 13, 2025 at 11:37 PM UTC, Sean Christopherson wrote:
> When testing for VMLAUNCH vs. VMRESUME, use the copy of @flags from the
> stack instead of first moving it to EBX, and then propagating
> VMX_RUN_VMRESUME to RFLAGS.CF (because RBX is clobbered with the guest
> value prior to the conditional branch to VMLAUNCH).  Stashing information
> in RFLAGS is gross, especially with the writer and reader being bifurcated
> by yet more gnarly assembly code.
>
> Opportunistically drop the SHIFT macros as they existed purely to allow
> the VM-Enter flow to use Bit Test.
>
> Suggested-by: Borislav Petkov <bp@...en8.de>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Brendan Jackman <jackmanb@...gle.com>