linux-kernel - Re: [PATCH] KVM: x86: Use "checked" versions of get_user() and put

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Message-ID: <176305665095.1602845.9274238002903506152.b4-ty@google.com>
Date: Thu, 13 Nov 2025 16:46:03 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, 
	Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] KVM: x86: Use "checked" versions of get_user() and put_user()

On Thu, 06 Nov 2025 13:02:06 -0800, Sean Christopherson wrote:
> Use the normal, checked versions for get_user() and put_user() instead of
> the double-underscore versions that omit range checks, as the checked
> versions are actually measurably faster on modern CPUs (12%+ on Intel,
> 25%+ on AMD).
> 
> The performance hit on the unchecked versions is almost entirely due to
> the added LFENCE on CPUs where LFENCE is serializing (which is effectively
> all modern CPUs), which was added by commit 304ec1b05031 ("x86/uaccess:
> Use __uaccess_begin_nospec() and uaccess_try_nospec").  The small
> optimizations done by commit b19b74bc99b1 ("x86/mm: Rework address range
> check in get_user() and put_user()") likely shave a few cycles off, but
> the bulk of the extra latency comes from the LFENCE.
> 
> [...]

Applied to kvm-x86 misc, with a call out in the changelog that the Hyper-V
path isn't performance sensitive, and that the motiviation is consistency and
the purging of __{get,put}_user().

[1/1] KVM: x86: Use "checked" versions of get_user() and put_user()
      https://github.com/kvm-x86/linux/commit/d1bc00483759

--
https://github.com/kvm-x86/linux/tree/next