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

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Message-ID: <18c8bccc-f57f-401c-9b86-248d1632f9d9@citrix.com>
Date: Wed, 11 Feb 2026 16:46:41 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>,
 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 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.

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

~Andrew