From: Sean Christopherson <seanjc@google.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: David Laight <david.laight.linux@gmail.com>,
ubizjak@gmail.com, bp@alien8.de, dave.hansen@linux.intel.com,
hpa@zytor.com, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org, mingo@kernel.org,
pbonzini@redhat.com, tglx@kernel.org, x86@kernel.org
Subject: Re: [PATCH 1/2] KVM: VMX: Drop obsolete branch hint prefixes from inline asm
Date: Wed, 11 Feb 2026 08:49:06 -0800 [thread overview]
Message-ID: <aYyzAk9yCOpDWpV6@google.com> (raw)
In-Reply-To: <18c8bccc-f57f-401c-9b86-248d1632f9d9@citrix.com>
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
next prev parent reply other threads:[~2026-02-11 16:49 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-02-11 10:28 [PATCH 1/2] KVM: VMX: Drop obsolete branch hint prefixes from inline asm Uros Bizjak
2026-02-11 10:28 ` [PATCH 2/2] KVM: VMX: Use ASM_INPUT_RM in __vmcs_writel Uros Bizjak
2026-02-25 19:02 ` Nathan Chancellor
2026-02-11 10:57 ` [PATCH 1/2] KVM: VMX: Drop obsolete branch hint prefixes from inline asm Andrew Cooper
2026-02-11 13:36 ` Sean Christopherson
2026-02-11 13:43 ` David Laight
2026-02-11 13:55 ` Andrew Cooper
2026-02-11 15:44 ` Sean Christopherson
2026-02-11 16:11 ` Andrew Cooper
2026-02-11 16:17 ` Sean Christopherson
2026-02-11 16:46 ` Andrew Cooper
2026-02-11 16:49 ` Sean Christopherson [this message]
2026-02-11 21:35 ` David Laight
2026-03-05 17:08 ` Sean Christopherson
-- strict thread matches above, loose matches on Subject: below --
2026-02-12 5:33 Christian Ludloff
2026-02-12 16:18 ` Andrew Cooper
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