Re: [PATCH 13/15] KVM: SVM: Add and use svm_register_cache_reset()

Paolo Bonzini
2021-11-18T17:55:16Z

On 11/18/21 17:28, Lai Jiangshan wrote:
> Using VMX_REGS_DIRTY_SET and SVM_REGS_DIRTY_SET and making the code
> similar is my intent for patch12,13.  If it causes confusing, I would
> like to make a second thought.  SVM_REGS_DIRTY_SET does be special
> in svm where VCPU_EXREG_CR3 is in it by definition, but it is not
> added into SVM_REGS_DIRTY_SET in the patch just for optimization to allow
> the compiler optimizes the line of code out.

I think this is where we disagree. In my opinion it is enough to
document that CR3 _can_ be out of date, but it doesn't have to be marked
dirty because its dirty bit is effectively KVM_REQ_LOAD_MMU_PGD.

For VMX, it is important to clear VCPU_EXREG_CR3 because the combination
"avail=0, dirty=1" is nonsensical:

av d
0 0 in VMCS
0 1 *INVALID*
1 0 in vcpu->arch
1 1 in vcpu->arch, needs store

But on SVM, VCPU_EXREG_CR3 is always available.

Thinking more about it, it makes more sense for VMX to reset _all_
bits of dirty to 0, just like it was before your change, but doing
so even earlier in vmx_vcpu_run.

I appreciate that VMX_REGS_LAZY_UPDATE_SET is useful for documentation,
but it's also important that the values in avail/dirty make sense as
a pair.

So here is what I would do:

diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
index 6e6d0d01f18d..ac3d3bd662f4 100644
--- a/arch/x86/kvm/kvm_cache_regs.h
+++ b/arch/x86/kvm/kvm_cache_regs.h
@@ -43,6 +43,13 @@ BUILD_KVM_GPR_ACCESSORS(r14, R14)
BUILD_KVM_GPR_ACCESSORS(r15, R15)
#endif

+/*
+ * avail dirty
+ * 0 0 register in VMCS/VMCB
+ * 0 1 *INVALID*
+ * 1 0 register in vcpu->arch
+ * 1 1 register in vcpu->arch, needs to be stored back
+ */
static inline bool kvm_register_is_available(struct kvm_vcpu *vcpu,
enum kvm_reg reg)
{
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 6fce61fc98e3..72ae67e214b5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6635,6 +6635,7 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
vmcs_writel(GUEST_RSP, vcpu->arch.regs[VCPU_REGS_RSP]);
if (kvm_register_is_dirty(vcpu, VCPU_REGS_RIP))
vmcs_writel(GUEST_RIP, vcpu->arch.regs[VCPU_REGS_RIP]);
+ vcpu->arch.regs_dirty = 0;

cr3 = __get_current_cr3_fast();
if (unlikely(cr3 != vmx->loaded_vmcs->host_state.cr3)) {
@@ -6729,7 +6730,7 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
loadsegment(es, __USER_DS);
#endif

- vmx_register_cache_reset(vcpu);
+ vcpu->arch.regs_avail &= ~VMX_REGS_LAZY_LOAD_SET;

pt_guest_exit(vmx);

diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index 4df2ac24ffc1..f978699480e3 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -473,19 +473,21 @@ BUILD_CONTROLS_SHADOW(pin, PIN_BASED_VM_EXEC_CONTROL)
BUILD_CONTROLS_SHADOW(exec, CPU_BASED_VM_EXEC_CONTROL)
BUILD_CONTROLS_SHADOW(secondary_exec, SECONDARY_VM_EXEC_CONTROL)

-static inline void vmx_register_cache_reset(struct kvm_vcpu *vcpu)
-{
- vcpu->arch.regs_avail = ~((1 << VCPU_REGS_RIP) | (1 << VCPU_REGS_RSP)
- | (1 << VCPU_EXREG_RFLAGS)
- | (1 << VCPU_EXREG_PDPTR)
- | (1 << VCPU_EXREG_SEGMENTS)
- | (1 << VCPU_EXREG_CR0)
- | (1 << VCPU_EXREG_CR3)
- | (1 << VCPU_EXREG_CR4)
- | (1 << VCPU_EXREG_EXIT_INFO_1)
- | (1 << VCPU_EXREG_EXIT_INFO_2));
- vcpu->arch.regs_dirty = 0;
-}
+/*
+ * VMX_REGS_LAZY_LOAD_SET - The set of registers that will be updated in the
+ * cache on demand. Other registers not listed here are synced to
+ * the cache immediately after VM-Exit.
+ */
+#define VMX_REGS_LAZY_LOAD_SET ((1 << VCPU_REGS_RIP) | \
+ (1 << VCPU_REGS_RSP) | \
+ (1 << VCPU_EXREG_RFLAGS) | \
+ (1 << VCPU_EXREG_PDPTR) | \
+ (1 << VCPU_EXREG_SEGMENTS) | \
+ (1 << VCPU_EXREG_CR0) | \
+ (1 << VCPU_EXREG_CR3) | \
+ (1 << VCPU_EXREG_CR4) | \
+ (1 << VCPU_EXREG_EXIT_INFO_1) | \
+ (1 << VCPU_EXREG_EXIT_INFO_2))

static inline struct kvm_vmx *to_kvm_vmx(struct kvm *kvm)
{

and likewise for SVM:

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index eb2a2609cae8..4b22aa7d55d0 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3944,6 +3944,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
vcpu->arch.regs[VCPU_REGS_RSP] = svm->vmcb->save.rsp;
vcpu->arch.regs[VCPU_REGS_RIP] = svm->vmcb->save.rip;
}
+ vcpu->arch.regs_dirty = 0;

if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
kvm_before_interrupt(vcpu);
@@ -3978,7 +3978,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
vcpu->arch.apf.host_apf_flags =
kvm_read_and_reset_apf_flags();

- kvm_register_clear_available(vcpu, VCPU_EXREG_PDPTR);
+ vcpu->arch.regs_avail &= ~SVM_REGS_LAZY_LOAD_SET;

/*
* We need to handle MC intercepts here before the vcpu has a chance to
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 32769d227860..b3c3c3098216 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -321,6 +321,16 @@ static inline bool vmcb_is_dirty(struct vmcb *vmcb, int bit)
return !test_bit(bit, (unsigned long *)&vmcb->control.clean);
}

+/*
+ * Only the PDPTRs are loaded on demand into the shadow MMU. All other
+ * fields are synchronized in handle_exit, because accessing the VMCB is cheap.
+ *
+ * CR3 might be out of date in the VMCB but it is not marked dirty; instead,
+ * KVM_REQ_LOAD_MMU_PGD is always requested when the cached vcpu->arch.cr3
+ * is changed. svm_load_mmu_pgd() then syncs the new CR3 value into the VMCB.
+ */
+#define SVM_REGS_LAZY_LOAD_SET (1 << VCPU_EXREG_PDPTR)
+
static inline struct vcpu_svm *to_svm(struct kvm_vcpu *vcpu)
{
return container_of(vcpu, struct vcpu_svm, vcpu);