From: Robert Hoo <robert.hoo.linux@gmail.com>
To: Mathias Krause <minipli@grsecurity.net>, kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org,
Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v4 2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled
Date: Sun, 7 May 2023 15:32:21 +0800 [thread overview]
Message-ID: <e70af22a-f09f-aadb-8353-35b29d2def61@gmail.com> (raw)
In-Reply-To: <20230322013731.102955-3-minipli@grsecurity.net>
On 3/22/2023 9:37 AM, Mathias Krause wrote:
> There is no need to unload the MMU roots with TDP enabled when only
> CR0.WP has changed -- the paging structures are still valid, only the
> permission bitmap needs to be updated.
>
> One heavy user of toggling CR0.WP is grsecurity's KERNEXEC feature to
> implement kernel W^X.
>
> The optimization brings a huge performance gain for this case as the
> following micro-benchmark running 'ssdd 10 50000' from rt-tests[1] on a
> grsecurity L1 VM shows (runtime in seconds, lower is better):
>
> legacy TDP shadow
> kvm-x86/next@d8708b 8.43s 9.45s 70.3s
> +patch 5.39s 5.63s 70.2s
>
> For legacy MMU this is ~36% faster, for TTP MMU even ~40% faster.
TTP --> TDP
> void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0, unsigned long cr0)
> {
> + /*
> + * CR0.WP is incorporated into the MMU role, but only for non-nested,
> + * indirect shadow MMUs. If TDP is enabled, the MMU's metadata needs
> + * to be updated, e.g. so that emulating guest translations does the
> + * right thing, but there's no need to unload the root as CR0.WP
> + * doesn't affect SPTEs.
> + */
> + if (tdp_enabled && (cr0 ^ old_cr0) == X86_CR0_WP) {
Curiously, this patch only affects tdp_enabled, why does legacy MMU also
see comparable performance gains?
next prev parent reply other threads:[~2023-05-07 7:36 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-22 1:37 [PATCH v4 0/6] KVM: MMU: performance tweaks for heavy CR0.WP users Mathias Krause
2023-03-22 1:37 ` [PATCH v4 1/6] KVM: x86/mmu: Avoid indirect call for get_cr3 Mathias Krause
2023-03-22 1:37 ` [PATCH v4 2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled Mathias Krause
2023-05-07 7:32 ` Robert Hoo [this message]
2023-05-08 9:30 ` Mathias Krause
2023-05-09 1:04 ` Robert Hoo
2023-03-22 1:37 ` [PATCH v4 3/6] KVM: x86: Ignore CR0.WP toggles in non-paging mode Mathias Krause
2023-03-22 1:37 ` [PATCH v4 4/6] KVM: x86: Make use of kvm_read_cr*_bits() when testing bits Mathias Krause
2023-03-22 1:37 ` [PATCH v4 5/6] KVM: x86/mmu: Fix comment typo Mathias Krause
2023-03-22 1:37 ` [PATCH v4 6/6] KVM: VMX: Make CR0.WP a guest owned bit Mathias Krause
2023-03-27 8:33 ` Xiaoyao Li
2023-03-27 8:37 ` Mathias Krause
2023-03-27 13:48 ` Xiaoyao Li
2023-03-30 8:45 ` Mathias Krause
2023-03-30 17:12 ` Sean Christopherson
2023-03-30 20:15 ` Mathias Krause
2023-03-30 20:30 ` Mathias Krause
2023-03-30 20:36 ` Sean Christopherson
2023-03-30 20:33 ` Sean Christopherson
2023-03-30 20:55 ` Mathias Krause
2023-03-31 14:18 ` Mathias Krause
2023-03-22 7:41 ` [PATCH v4 0/6] KVM: MMU: performance tweaks for heavy CR0.WP users Mathias Krause
2023-03-23 22:50 ` Sean Christopherson
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