lkml.org 
[lkml]   [2023]   [May]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    SubjectRe: [PATCH v4 2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled
    From
    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?

    \
     
     \ /
      Last update: 2023-05-07 09:33    [W:3.209 / U:0.972 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site