KVM: x86: Load guest/host PKRU outside of the fastpath run loop

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Move KVM's swapping of PKRU outside of the fastpath loop, as there is no
KVM code anywhere in the fastpath that accesses guest/userspace memory,
i.e. that can consume protection keys.

As documented by commit 1be0e61c1f ("KVM, pkeys: save/restore PKRU when
guest/host switches"), KVM just needs to ensure the host's PKRU is loaded
when KVM (or the kernel at-large) may access userspace memory.  And at the
time of commit 1be0e61c1f, KVM didn't have a fastpath, and PKU was
strictly contained to VMX, i.e. there was no reason to swap PKRU outside
of vmx_vcpu_run().

Over time, the "need" to swap PKRU close to VM-Enter was likely falsely
solidified by the association with XFEATUREs in commit 37486135d3
("KVM: x86: Fix pkru save/restore when guest CR4.PKE=0, move it to x86.c"),
and XFEATURE swapping was in turn moved close to VM-Enter/VM-Exit as a
KVM hack-a-fix ution for an #MC handler bug by commit 1811d979c7
("x86/kvm: move kvm_load/put_guest_xcr0 into atomic context").

Deferring the PKRU loads shaves ~40 cycles off the fastpath for Intel,
and ~60 cycles for AMD.  E.g. using INVD in KVM-Unit-Test's vmexit.c,
with extra hacks to enable CR4.PKE and PKRU=(-1u & ~0x3), latency numbers
for AMD Turin go from ~1560 => ~1500, and for Intel Emerald Rapids, go
from ~810 => ~770.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Jon Kohler <jon@nutanix.com>
Link: https://patch.msgid.link/20251118222328.2265758-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

...

This commit is contained in:

Sean Christopherson 2025-11-18 14:23:28 -08:00

parent 75c69c82f2

commit 7649412af3

4 changed files with 10 additions and 12 deletions

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2

arch/x86/kvm/svm/svm.c

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4

arch/x86/kvm/vmx/vmx.c

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arch/x86/kvm/x86.c

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arch/x86/kvm/x86.h

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