Commit b7ed46b2 authored by Sean Christopherson's avatar Sean Christopherson
Browse files

KVM: x86/mmu: Don't force flush if SPTE update clears Accessed bit 



Don't force a TLB flush if mmu_spte_update() clears the Accessed bit, as
access tracking tolerates false negatives, as evidenced by the
mmu_notifier hooks that explicitly test and age SPTEs without doing a TLB
flush.

In practice, this is very nearly a nop.  spte_write_protect() and
spte_clear_dirty() never clear the Accessed bit.  make_spte() always
sets the Accessed bit for !prefetch scenarios.  FNAME(sync_spte) only sets
SPTE if the protection bits are changing, i.e. if a flush will be needed
regardless of the Accessed bits.  And FNAME(pte_prefetch) sets SPTE if and
only if the old SPTE is !PRESENT.

That leaves kvm_arch_async_page_ready() as the one path that will generate
a !ACCESSED SPTE *and* overwrite a PRESENT SPTE.  And that's very arguably
a bug, as clobbering a valid SPTE in that case is nonsensical.

Tested-by: default avatarAlex Bennée <alex.bennee@linaro.org>
Link: https://lore.kernel.org/r/20241011021051.1557902-5-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 0387d79e

arch/x86/kvm/mmu/mmu.c

+9 −21
Original line number Diff line number Diff line
@@ -521,36 +521,24 @@ static u64 mmu_spte_update_no_track(u64 *sptep, u64 new_spte) 
 * not whether or not SPTEs were modified, i.e. only the write-tracking case

 * needs to flush at the time the SPTEs is modified, before dropping mmu_lock.

 *

 * Remote TLBs also need to be flushed if the Dirty bit is cleared, as false

 * negatives are not acceptable, e.g. if KVM is using D-bit based PML on VMX.

 *

 * Don't flush if the Accessed bit is cleared, as access tracking tolerates

 * false negatives, and the one path that does care about TLB flushes,

 * kvm_mmu_notifier_clear_flush_young(), uses mmu_spte_update_no_track().

 *

 * Returns true if the TLB needs to be flushed

 */

static bool mmu_spte_update(u64 *sptep, u64 new_spte)

{

	bool flush = false;

	u64 old_spte = mmu_spte_update_no_track(sptep, new_spte);



	if (!is_shadow_present_pte(old_spte))

		return false;



	/*

	 * For the spte updated out of mmu-lock is safe, since

	 * we always atomically update it, see the comments in

	 * spte_has_volatile_bits().

	 */

	if (is_mmu_writable_spte(old_spte) && !is_mmu_writable_spte(new_spte))

		flush = true;



	/*

	 * Flush TLB when accessed/dirty states are changed in the page tables,

	 * to guarantee consistency between TLB and page tables.

	 */



	if (is_accessed_spte(old_spte) && !is_accessed_spte(new_spte))

		flush = true;



	if (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte))

		flush = true;



	return flush;

	return (is_mmu_writable_spte(old_spte) && !is_mmu_writable_spte(new_spte)) ||

	       (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte));

}



/*