linux-kernel - Re: [PATCH 1/2] KVM: VMX: Drop obsolete branch hint prefixes from inline asm

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Message-ID: <aYyzAk9yCOpDWpV6@google.com>
Date: Wed, 11 Feb 2026 08:49:06 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: David Laight <david.laight.linux@...il.com>, ubizjak@...il.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, hpa@...or.com, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, mingo@...nel.org, pbonzini@...hat.com, 
	tglx@...nel.org, x86@...nel.org
Subject: Re: [PATCH 1/2] KVM: VMX: Drop obsolete branch hint prefixes from
 inline asm

On Wed, Feb 11, 2026, Andrew Cooper wrote:
> On 11/02/2026 4:17 pm, Sean Christopherson wrote:
> > On Wed, Feb 11, 2026, Andrew Cooper wrote:
> >> On 11/02/2026 3:44 pm, Sean Christopherson wrote:
> >>> On Wed, Feb 11, 2026, Andrew Cooper wrote:
> >> This change is almost certainly marginal at best.  It's not as if
> >> VMREAD/VMWRITE lead to good code gen even at the best of times.
> > Yeah, but adding in them in the first place was even more marginal (I added the
> > hints as much for documentation purposes as anything else).  Absent proof that
> > having the hints is a net positive, I'm inclined to trust the compiler folks on
> > what is/isn't optimal, and drop them.
> 
> Branch mispredicts in the P4 could easily eat up 150 cycles before the
> frontend got it's act together.

Heh, I'm willing to risk getting yelled at by the one person running KVM on P4.

> However, optimising VMREAD/VMWRITE and not the whole kernel seems
> somewhat futile.
> 
> ~Andrew