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KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled

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[ Upstream commit 01b31714bd ]

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 TDP MMU even ~40% faster. Also
TDP and legacy MMU now both have a similar runtime which vanishes the
need to disable TDP MMU for grsecurity.

Shadow MMU sees no measurable difference and is still slow, as expected.

[1] https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git

Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20230322013731.102955-3-minipli@grsecurity.net
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Mathias Krause 2023-05-12 15:20:21 +02:00 committed by Greg Kroah-Hartman
parent b1b91f9704
commit 65c29d3941
1 changed files with 12 additions and 0 deletions
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12
arch/x86/kvm/x86.c
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@ -906,6 +906,18 @@ EXPORT_SYMBOL_GPL(load_pdptrs);
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) {
kvm_init_mmu(vcpu);
return;
}
if ((cr0 ^ old_cr0) & X86_CR0_PG) {
kvm_clear_async_pf_completion_queue(vcpu);
kvm_async_pf_hash_reset(vcpu);