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author | Mathias Krause <minipli@grsecurity.net> | 2023-03-22 04:37:27 +0300 |
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committer | Sean Christopherson <seanjc@google.com> | 2023-03-22 17:47:24 +0300 |
commit | 01b31714bd90be2784f7145bf93b7f78f3d081e1 (patch) | |
tree | ce1bbb374233c1ba261a1ed5ed2264d3c05ed56c /arch/x86/kvm/x86.c | |
parent | d8708b80fa0e6e21bc0c9e7276ad0bccef73b6e7 (diff) | |
download | linux-01b31714bd90be2784f7145bf93b7f78f3d081e1.tar.xz |
KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled
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>
Diffstat (limited to 'arch/x86/kvm/x86.c')
-rw-r--r-- | arch/x86/kvm/x86.c | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 237c483b1230..c6d909778b2c 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -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); |