[PATCH 8/8] KVM: nSVM: read only changed fields of the nested guest data area

Maxim Levitsky
2020-08-20T09:15:13Z

This allows us to only read fields that are marked as dirty by the nested
guest on vmentry.

I doubt that this has any perf impact but this way it is a bit closer
to real hardware.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/svm/nested.c | 58 +++++++++++++++++++++++++--------------
arch/x86/kvm/svm/svm.c | 2 +-
arch/x86/kvm/svm/svm.h | 5 ++++
3 files changed, 44 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index acc4b26fcfcc..f3eef48caee6 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -266,40 +266,57 @@ static void load_nested_vmcb_control(struct vcpu_svm *svm,
}

static void load_nested_vmcb_save(struct vcpu_svm *svm,
- struct vmcb_save_area *save)
+ struct vmcb_save_area *save,
+ u32 clean)
{
svm->nested.vmcb->save.rflags = save->rflags;
svm->nested.vmcb->save.rax = save->rax;
svm->nested.vmcb->save.rsp = save->rsp;
svm->nested.vmcb->save.rip = save->rip;

- svm->nested.vmcb->save.es = save->es;
- svm->nested.vmcb->save.cs = save->cs;
- svm->nested.vmcb->save.ss = save->ss;
- svm->nested.vmcb->save.ds = save->ds;
- svm->nested.vmcb->save.cpl = save->cpl;
+ if (is_dirty(clean, VMCB_SEG)) {
+ svm->nested.vmcb->save.es = save->es;
+ svm->nested.vmcb->save.cs = save->cs;
+ svm->nested.vmcb->save.ss = save->ss;
+ svm->nested.vmcb->save.ds = save->ds;
+ svm->nested.vmcb->save.cpl = save->cpl;
+ }

- svm->nested.vmcb->save.gdtr = save->gdtr;
- svm->nested.vmcb->save.idtr = save->idtr;
+ if (is_dirty(clean, VMCB_DT)) {
+ svm->nested.vmcb->save.gdtr = save->gdtr;
+ svm->nested.vmcb->save.idtr = save->idtr;
+ }

- svm->nested.vmcb->save.efer = save->efer;
- svm->nested.vmcb->save.cr3 = save->cr3;
- svm->nested.vmcb->save.cr4 = save->cr4;
- svm->nested.vmcb->save.cr0 = save->cr0;
+ if (is_dirty(clean, VMCB_CR)) {
+ svm->nested.vmcb->save.efer = save->efer;
+ svm->nested.vmcb->save.cr3 = save->cr3;
+ svm->nested.vmcb->save.cr4 = save->cr4;
+ svm->nested.vmcb->save.cr0 = save->cr0;
+ }

- svm->nested.vmcb->save.cr2 = save->cr2;
+ if (is_dirty(clean, VMCB_CR2))
+ svm->nested.vmcb->save.cr2 = save->cr2;

- svm->nested.vmcb->save.dr7 = save->dr7;
- svm->nested.vmcb->save.dr6 = save->dr6;
+ if (is_dirty(clean, VMCB_DR)) {
+ svm->nested.vmcb->save.dr7 = save->dr7;
+ svm->nested.vmcb->save.dr6 = save->dr6;
+ }

- svm->nested.vmcb->save.g_pat = save->g_pat;
+ if ((clean & VMCB_NPT) == 0)
+ svm->nested.vmcb->save.g_pat = save->g_pat;
}

void load_nested_vmcb(struct vcpu_svm *svm, struct vmcb *nested_vmcb, u64 vmcb_gpa)
{
- svm->nested.vmcb_gpa = vmcb_gpa;
+ u32 clean = nested_vmcb->control.clean;
+
+ if (svm->nested.vmcb_gpa != vmcb_gpa) {
+ svm->nested.vmcb_gpa = vmcb_gpa;
+ clean = 0;
+ }
+
load_nested_vmcb_control(svm, &nested_vmcb->control);
- load_nested_vmcb_save(svm, &nested_vmcb->save);
+ load_nested_vmcb_save(svm, &nested_vmcb->save, clean);
}

/*
@@ -619,7 +636,6 @@ int nested_svm_vmexit(struct vcpu_svm *svm)

/* Exit Guest-Mode */
leave_guest_mode(&svm->vcpu);
- svm->nested.vmcb_gpa = 0;
WARN_ON_ONCE(svm->nested.nested_run_pending);

/* in case we halted in L2 */
@@ -676,7 +692,7 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
* Note: since CPU might have changed the values we can't
* trust clean bits
*/
- load_nested_vmcb_save(svm, &nested_vmcb->save);
+ load_nested_vmcb_save(svm, &nested_vmcb->save, 0);

/* Restore the original control entries */
copy_vmcb_control_area(&vmcb->control, &hsave->control);
@@ -759,6 +775,7 @@ int svm_allocate_nested(struct vcpu_svm *svm)
goto free_page3;

svm->nested.vmcb = page_address(vmcb_page);
+ svm->nested.vmcb_gpa = U64_MAX;
clear_page(svm->nested.vmcb);

svm->nested.initialized = true;
@@ -785,6 +802,7 @@ void svm_free_nested(struct vcpu_svm *svm)

__free_page(virt_to_page(svm->nested.vmcb));
svm->nested.vmcb = NULL;
+ svm->nested.vmcb_gpa = U64_MAX;

svm->nested.initialized = false;
}
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 06668e0f93e7..f0bb7f622dca 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3924,7 +3924,7 @@ static int svm_pre_leave_smm(struct kvm_vcpu *vcpu, const char *smstate)
if (kvm_vcpu_map(&svm->vcpu, gpa_to_gfn(vmcb_gpa), &map) == -EINVAL)
return 1;

- load_nested_vmcb(svm, map.hva, vmcb);
+ load_nested_vmcb(svm, map.hva, vmcb_gpa);
ret = enter_svm_guest_mode(svm);

kvm_vcpu_unmap(&svm->vcpu, &map, true);
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 80231ef8de6f..4a383c519fdf 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -204,6 +204,11 @@ static inline void vmcb_mark_dirty(struct vmcb *vmcb, int bit)
vmcb->control.clean &= ~(1 << bit);
}

+static inline bool is_dirty(u32 clean, int bit)
+{
+ return (clean & (1 << bit)) == 0;
+}
+
static inline struct vcpu_svm *to_svm(struct kvm_vcpu *vcpu)
{
return container_of(vcpu, struct vcpu_svm, vcpu);
--
2.26.2