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KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs

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Introduce MMU_PRESENT to explicitly track which SPTEs are "present" from
the MMU's perspective.  Checking for shadow-present SPTEs is a very
common operation for the MMU, particularly in hot paths such as page
faults.  With the addition of "removed" SPTEs for the TDP MMU,
identifying shadow-present SPTEs is quite costly especially since it
requires checking multiple 64-bit values.

On 64-bit KVM, this reduces the footprint of kvm.ko's .text by ~2k bytes.
On 32-bit KVM, this increases the footprint by ~200 bytes, but only
because gcc now inlines several more MMU helpers, e.g. drop_parent_pte().

We now need to drop bit 11, used for the MMU_PRESENT flag, from
the set of bits used to store the generation number in MMIO SPTEs.
Otherwise MMIO SPTEs with bit 11 set would get false positives for
is_shadow_present_spte() and lead to a variety of fireworks, from oopses
to likely hangs of the host kernel.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-21-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson
2021-02-25 12:47:45 -08:00
committed by Paolo Bonzini
parent 613a3f3797
commit edea7c4fc2
2 changed files with 21 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

8
arch/x86/kvm/mmu/spte.c
View File

@@ -94,7 +94,7 @@ int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
bool can_unsync, bool host_writable, bool ad_disabled,
u64 *new_spte)
{
u64 spte = 0;
u64 spte = SPTE_MMU_PRESENT_MASK;
int ret = 0;
if (ad_disabled)
@@ -183,10 +183,10 @@ out:
u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled)
{
u64 spte;
u64 spte = SPTE_MMU_PRESENT_MASK;
spte = __pa(child_pt) | shadow_present_mask | PT_WRITABLE_MASK |
shadow_user_mask | shadow_x_mask | shadow_me_mask;
spte |= __pa(child_pt) | shadow_present_mask | PT_WRITABLE_MASK |
shadow_user_mask | shadow_x_mask | shadow_me_mask;
if (ad_disabled)
spte |= SPTE_TDP_AD_DISABLED_MASK;

23
arch/x86/kvm/mmu/spte.h
View File

@@ -5,6 +5,15 @@
#include "mmu_internal.h"
/*
* A MMU present SPTE is backed by actual memory and may or may not be present
* in hardware. E.g. MMIO SPTEs are not considered present. Use bit 11, as it
* is ignored by all flavors of SPTEs and checking a low bit often generates
* better code than for a high bit, e.g. 56+. MMU present checks are pervasive
* enough that the improved code generation is noticeable in KVM's footprint.
*/
#define SPTE_MMU_PRESENT_MASK BIT_ULL(11)
/*
* TDP SPTES (more specifically, EPT SPTEs) may not have A/D bits, and may also
* be restricted to using write-protection (for L2 when CPU dirty logging, i.e.
@@ -92,11 +101,11 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
#undef SHADOW_ACC_TRACK_SAVED_MASK
/*
* Due to limited space in PTEs, the MMIO generation is a 20 bit subset of
* Due to limited space in PTEs, the MMIO generation is a 19 bit subset of
* the memslots generation and is derived as follows:
*
* Bits 0-8 of the MMIO generation are propagated to spte bits 3-11
* Bits 9-19 of the MMIO generation are propagated to spte bits 52-62
* Bits 0-7 of the MMIO generation are propagated to spte bits 3-10
* Bits 8-18 of the MMIO generation are propagated to spte bits 52-62
*
* The KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS flag is intentionally not included in
* the MMIO generation number, as doing so would require stealing a bit from
@@ -107,7 +116,7 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
*/
#define MMIO_SPTE_GEN_LOW_START 3
#define MMIO_SPTE_GEN_LOW_END 11
#define MMIO_SPTE_GEN_LOW_END 10
#define MMIO_SPTE_GEN_HIGH_START 52
#define MMIO_SPTE_GEN_HIGH_END 62
@@ -116,12 +125,14 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
MMIO_SPTE_GEN_LOW_START)
#define MMIO_SPTE_GEN_HIGH_MASK GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
MMIO_SPTE_GEN_HIGH_START)
static_assert(!(SPTE_MMU_PRESENT_MASK &
(MMIO_SPTE_GEN_LOW_MASK | MMIO_SPTE_GEN_HIGH_MASK)));
#define MMIO_SPTE_GEN_LOW_BITS (MMIO_SPTE_GEN_LOW_END - MMIO_SPTE_GEN_LOW_START + 1)
#define MMIO_SPTE_GEN_HIGH_BITS (MMIO_SPTE_GEN_HIGH_END - MMIO_SPTE_GEN_HIGH_START + 1)
/* remember to adjust the comment above as well if you change these */
static_assert(MMIO_SPTE_GEN_LOW_BITS == 9 && MMIO_SPTE_GEN_HIGH_BITS == 11);
static_assert(MMIO_SPTE_GEN_LOW_BITS == 8 && MMIO_SPTE_GEN_HIGH_BITS == 11);
#define MMIO_SPTE_GEN_LOW_SHIFT (MMIO_SPTE_GEN_LOW_START - 0)
#define MMIO_SPTE_GEN_HIGH_SHIFT (MMIO_SPTE_GEN_HIGH_START - MMIO_SPTE_GEN_LOW_BITS)
@@ -241,7 +252,7 @@ static inline bool is_access_track_spte(u64 spte)
static inline bool is_shadow_present_pte(u64 pte)
{
return (pte != 0) && !is_mmio_spte(pte) && !is_removed_spte(pte);
return !!(pte & SPTE_MMU_PRESENT_MASK);
}
static inline bool is_large_pte(u64 pte)