lkml.org 
[lkml]   [2020]   [Jan]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH v2 2/2] KVM: x86/mmu: Micro-optimize nEPT's bad memptype/XWR checks
Date
Rework the handling of nEPT's bad memtype/XWR checks to micro-optimize
the checks as much as possible. Move the check to a separate helper,
__is_bad_mt_xwr(), which allows the guest_rsvd_check usage in
paging_tmpl.h to omit the check entirely for paging32/64 (bad_mt_xwr is
always zero for non-nEPT) while retaining the bitwise-OR of the current
code for the shadow_zero_check in walk_shadow_page_get_mmio_spte().

Add a comment for the bitwise-OR usage in the mmio spte walk to avoid
future attempts to "fix" the code, which is what prompted this
optimization in the first place[*].

Opportunistically remove the superfluous '!= 0' and parantheses, and
use BIT_ULL() instead of open coding its equivalent.

The net effect is that code generation is largely unhanged for
walk_shadow_page_get_mmio_spte(), marginally better for
ept_prefetch_invalid_gpte(), and significantly improved for
paging32/64_prefetch_invalid_gpte().

Note, walk_shadow_page_get_mmio_spte() can't use a templated version of
the memtype/XRW as it works on the host's shadow PTEs, e.g. checks that
KVM hasn't borked its EPT tables. Even if it could be templated, the
benefits of having a single implementation far outweight the few uops
that would be saved for NPT or non-TDP paging, e.g. most compilers
inline it all the way to up kvm_mmu_page_fault().

[*] https://lkml.kernel.org/r/20200108001859.25254-1-sean.j.christopherson@intel.com

Cc: Jim Mattson <jmattson@google.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
arch/x86/kvm/mmu/mmu.c | 26 ++++++++++++++------------
arch/x86/kvm/mmu/paging_tmpl.h | 19 +++++++++++++++++--
2 files changed, 31 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 7269130ea5e2..2992ff7b42a7 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3968,20 +3968,14 @@ static gpa_t nonpaging_gva_to_gpa_nested(struct kvm_vcpu *vcpu, gpa_t vaddr,
static bool
__is_rsvd_bits_set(struct rsvd_bits_validate *rsvd_check, u64 pte, int level)
{
- int bit7 = (pte >> 7) & 1, low6 = pte & 0x3f;
+ int bit7 = (pte >> 7) & 1;

- return (pte & rsvd_check->rsvd_bits_mask[bit7][level-1]) |
- ((rsvd_check->bad_mt_xwr & (1ull << low6)) != 0);
+ return pte & rsvd_check->rsvd_bits_mask[bit7][level-1];
}

-static bool is_rsvd_bits_set(struct kvm_mmu *mmu, u64 gpte, int level)
+static bool __is_bad_mt_xwr(struct rsvd_bits_validate *rsvd_check, u64 pte)
{
- return __is_rsvd_bits_set(&mmu->guest_rsvd_check, gpte, level);
-}
-
-static bool is_shadow_zero_bits_set(struct kvm_mmu *mmu, u64 spte, int level)
-{
- return __is_rsvd_bits_set(&mmu->shadow_zero_check, spte, level);
+ return rsvd_check->bad_mt_xwr & BIT_ULL(pte & 0x3f);
}

static bool mmio_info_in_cache(struct kvm_vcpu *vcpu, u64 addr, bool direct)
@@ -4005,9 +3999,12 @@ walk_shadow_page_get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
{
struct kvm_shadow_walk_iterator iterator;
u64 sptes[PT64_ROOT_MAX_LEVEL], spte = 0ull;
+ struct rsvd_bits_validate *rsvd_check;
int root, leaf;
bool reserved = false;

+ rsvd_check = &vcpu->arch.mmu->shadow_zero_check;
+
walk_shadow_page_lockless_begin(vcpu);

for (shadow_walk_init(&iterator, vcpu, addr),
@@ -4022,8 +4019,13 @@ walk_shadow_page_get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
if (!is_shadow_present_pte(spte))
break;

- reserved |= is_shadow_zero_bits_set(vcpu->arch.mmu, spte,
- iterator.level);
+ /*
+ * Use a bitwise-OR instead of a logical-OR to aggregate the
+ * reserved bit and EPT's invalid memtype/XWR checks to avoid
+ * adding a Jcc in the loop.
+ */
+ reserved |= __is_bad_mt_xwr(rsvd_check, spte) |
+ __is_rsvd_bits_set(rsvd_check, spte, iterator.level);
}

walk_shadow_page_lockless_end(vcpu);
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 1fde6a1c506d..eaa00c4daeb1 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -128,6 +128,21 @@ static inline int FNAME(is_present_gpte)(unsigned long pte)
#endif
}

+static bool FNAME(is_bad_mt_xwr)(struct rsvd_bits_validate *rsvd_check, u64 gpte)
+{
+#if PTTYPE != PTTYPE_EPT
+ return false;
+#else
+ return __is_bad_mt_xwr(rsvd_check, gpte);
+#endif
+}
+
+static bool FNAME(is_rsvd_bits_set)(struct kvm_mmu *mmu, u64 gpte, int level)
+{
+ return __is_rsvd_bits_set(&mmu->guest_rsvd_check, gpte, level) ||
+ FNAME(is_bad_mt_xwr)(&mmu->guest_rsvd_check, gpte);
+}
+
static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
pt_element_t __user *ptep_user, unsigned index,
pt_element_t orig_pte, pt_element_t new_pte)
@@ -183,7 +198,7 @@ static bool FNAME(prefetch_invalid_gpte)(struct kvm_vcpu *vcpu,
!(gpte & PT_GUEST_ACCESSED_MASK))
goto no_present;

- if (is_rsvd_bits_set(vcpu->arch.mmu, gpte, PT_PAGE_TABLE_LEVEL))
+ if (FNAME(is_rsvd_bits_set)(vcpu->arch.mmu, gpte, PT_PAGE_TABLE_LEVEL))
goto no_present;

return false;
@@ -400,7 +415,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
if (unlikely(!FNAME(is_present_gpte)(pte)))
goto error;

- if (unlikely(is_rsvd_bits_set(mmu, pte, walker->level))) {
+ if (unlikely(FNAME(is_rsvd_bits_set)(mmu, pte, walker->level))) {
errcode = PFERR_RSVD_MASK | PFERR_PRESENT_MASK;
goto error;
}
--
2.24.1
\
 
 \ /
  Last update: 2020-01-10 00:07    [W:0.079 / U:2.276 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site