From: Sean Christopherson <seanjc@google.com>
To: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Paolo Bonzini <pbonzini@redhat.com>,
Lai Jiangshan <laijs@linux.alibaba.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 4/4] KVM: X86: Only get rflags when needed in permission_fault()
Date: Tue, 7 Dec 2021 21:57:43 +0000 [thread overview]
Message-ID: <Ya/Y1+6BR4exkTKK@google.com> (raw)
In-Reply-To: <20211207095039.53166-5-jiangshanlai@gmail.com>
On Tue, Dec 07, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> In same cases, it doesn't need to get rflags for SMAP checks.
>
> For example: it is user mode access, it could have contained other
> permission fault, SMAP is not enabled, it is implicit supervisor
> access, or it is nested TDP pagetable.
I don't disagree that reading RFLAGS is silly and _may_ have worse performance,
but I'd prefer any change have actual numbers to justify that it's an improvement
or at least a wash / in the noise.
Too much of the MMU code (and KVM in general) has optimizations like this that
have bitrotted horribly over the years. And in many/most cases, the original commit
didn't provide performance numbers, so it's not even clear that the "optimizations"
were _ever_ a net win.
prev parent reply other threads:[~2021-12-07 21:57 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-07 9:50 [PATCH 0/4] KVM: X86: Improve permission_fault() for SMAP Lai Jiangshan
2021-12-07 9:50 ` [PATCH 1/4] KVM: X86: Fix comments in update_permission_bitmask Lai Jiangshan
2021-12-07 9:50 ` [PATCH 2/4] KVM: X86: Rename variable smap to not_smap in permission_fault() Lai Jiangshan
2021-12-07 9:50 ` [PATCH 3/4] KVM: X86: Handle implicit supervisor access with SMAP Lai Jiangshan
2021-12-07 21:52 ` Sean Christopherson
2021-12-07 23:30 ` Lai Jiangshan
2021-12-08 9:12 ` Paolo Bonzini
2021-12-07 9:50 ` [PATCH 4/4] KVM: X86: Only get rflags when needed in permission_fault() Lai Jiangshan
2021-12-07 21:57 ` Sean Christopherson [this message]
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