I've been reading the x86's mmu.c recently and had been wondering
about something. Avi's recent mmu documentation (thanks!) seems to
have confirmed my understanding of how the shadow paging is supposed
to be working. In TDP mode, when mmu_alloc_roots() calls
kvm_mmu_get_page(), why does it pass (vcpu->arch.cr3 >> PAGE_SHIFT) or
(vcpu->arch.mmu.pae_root[i]) as gfn?
It seems to me that in TDP mode, gfn should be either zero for the
root page table, or 0/1GB/2GB/3GB (for PAE page tables).
The existing behavior can lead to multiple, semantically-identical TDP
roots being created by mmu_alloc_roots, depending on the VCPU's CR3 at
the time that mmu_alloc_roots was called. But the nested page tables
should be* independent of the VCPU state. That wastes some memory and
causes extra page faults while populating the extra copies of the page
tables.
*assuming that we aren't modeling per-VCPU state that might change the
physical address map as seen by that VCPU, such as setting the APIC
base to an address overlapping RAM.
All feedback would be welcome, since I'm new to this system! A
strawman patch follows.
thanks,
-Eric
--
For TDP mode, avoid creating multiple page table roots for the single
guest-to-host physical address map by fixing the inputs used for the
shadow page table hash in mmu_alloc_roots().
Signed-off-by: Eric Northup <digitaleric@xxxxxxxxxx>
---
arch/x86/kvm/mmu.c | 12 ++++++++----
1 files changed, 8 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index ddfa865..9696d65 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2059,10 +2059,12 @@ static int mmu_alloc_roots(struct kvm_vcpu *vcpu)
hpa_t root = vcpu->arch.mmu.root_hpa;
ASSERT(!VALID_PAGE(root));
- if (tdp_enabled)
- direct = 1;
if (mmu_check_root(vcpu, root_gfn))
return 1;
+ if (tdp_enabled) {
+ direct = 1;
+ root_gfn = 0;
+ }
sp = kvm_mmu_get_page(vcpu, root_gfn, 0,
PT64_ROOT_LEVEL, direct,
ACC_ALL, NULL);
@@ -2072,8 +2074,6 @@ static int mmu_alloc_roots(struct kvm_vcpu *vcpu)
return 0;
}
direct = !is_paging(vcpu);
- if (tdp_enabled)
- direct = 1;
for (i = 0; i < 4; ++i) {
hpa_t root = vcpu->arch.mmu.pae_root[i];
@@ -2089,6 +2089,10 @@ static int mmu_alloc_roots(struct kvm_vcpu *vcpu)
root_gfn = 0;
if (mmu_check_root(vcpu, root_gfn))
return 1;
+ if (tdp_enabled) {
+ direct = 1;
+ root_gfn = i << 30;
+ }
sp = kvm_mmu_get_page(vcpu, root_gfn, i << 30,
PT32_ROOT_LEVEL, direct,
ACC_ALL, NULL);
--
--
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