[PATCH RFC 5/5] x86: KVM: svm: remove hardcoded instruction length from intercepts

Vitaly Kuznetsov
2019-06-20T11:02:57Z

Various intercepts hard-code the respective instruction lengths to optimize
skip_emulated_instruction(): when next_rip is pre-set we skip
kvm_emulate_instruction(vcpu, EMULTYPE_SKIP). The optimization is, however,
incorrect: different (redundant) prefixes could be used to enlarge the
instruction. We can't really avoid decoding.

svm->next_rip is not used when CPU supports 'nrips' (X86_FEATURE_NRIPS)
feature: next RIP is provided in VMCB. The feature is not really new
(Opteron G3s had it already) and the change should have zero affect.

Remove manual svm->next_rip setting with hard-coded instruction lengths. The
only case where we now use svm->next_rip is EXIT_IOIO: the instruction
length is provided to us by hardware.

Reported-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
arch/x86/kvm/svm.c | 11 -----------
1 file changed, 11 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 39e61029f401..4c29859fecde 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -2890,13 +2890,11 @@ static int nop_on_interception(struct vcpu_svm *svm)

static int halt_interception(struct vcpu_svm *svm)
{
- svm->next_rip = kvm_rip_read(&svm->vcpu) + 1;
return kvm_emulate_halt(&svm->vcpu);
}

static int vmmcall_interception(struct vcpu_svm *svm)
{
- svm->next_rip = kvm_rip_read(&svm->vcpu) + 3;
return kvm_emulate_hypercall(&svm->vcpu);
}

@@ -3684,7 +3682,6 @@ static int vmload_interception(struct vcpu_svm *svm)

nested_vmcb = map.hva;

- svm->next_rip = kvm_rip_read(&svm->vcpu) + 3;
ret = kvm_skip_emulated_instruction(&svm->vcpu);

nested_svm_vmloadsave(nested_vmcb, svm->vmcb);
@@ -3711,7 +3708,6 @@ static int vmsave_interception(struct vcpu_svm *svm)

nested_vmcb = map.hva;

- svm->next_rip = kvm_rip_read(&svm->vcpu) + 3;
ret = kvm_skip_emulated_instruction(&svm->vcpu);

nested_svm_vmloadsave(svm->vmcb, nested_vmcb);
@@ -3762,7 +3758,6 @@ static int stgi_interception(struct vcpu_svm *svm)
if (vgif_enabled(svm))
clr_intercept(svm, INTERCEPT_STGI);

- svm->next_rip = kvm_rip_read(&svm->vcpu) + 3;
ret = kvm_skip_emulated_instruction(&svm->vcpu);
kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);

@@ -3778,7 +3773,6 @@ static int clgi_interception(struct vcpu_svm *svm)
if (nested_svm_check_permissions(svm))
return 1;

- svm->next_rip = kvm_rip_read(&svm->vcpu) + 3;
ret = kvm_skip_emulated_instruction(&svm->vcpu);

disable_gif(svm);
@@ -3803,7 +3797,6 @@ static int invlpga_interception(struct vcpu_svm *svm)
/* Let's treat INVLPGA the same as INVLPG (can be optimized!) */
kvm_mmu_invlpg(vcpu, kvm_rax_read(&svm->vcpu));

- svm->next_rip = kvm_rip_read(&svm->vcpu) + 3;
return kvm_skip_emulated_instruction(&svm->vcpu);
}

@@ -3826,7 +3819,6 @@ static int xsetbv_interception(struct vcpu_svm *svm)
u32 index = kvm_rcx_read(&svm->vcpu);

if (kvm_set_xcr(&svm->vcpu, index, new_bv) == 0) {
- svm->next_rip = kvm_rip_read(&svm->vcpu) + 3;
return kvm_skip_emulated_instruction(&svm->vcpu);
}

@@ -3903,7 +3895,6 @@ static int task_switch_interception(struct vcpu_svm *svm)

static int cpuid_interception(struct vcpu_svm *svm)
{
- svm->next_rip = kvm_rip_read(&svm->vcpu) + 2;
return kvm_emulate_cpuid(&svm->vcpu);
}

@@ -4233,7 +4224,6 @@ static int rdmsr_interception(struct vcpu_svm *svm)

kvm_rax_write(&svm->vcpu, msr_info.data & 0xffffffff);
kvm_rdx_write(&svm->vcpu, msr_info.data >> 32);
- svm->next_rip = kvm_rip_read(&svm->vcpu) + 2;
return kvm_skip_emulated_instruction(&svm->vcpu);
}
}
@@ -4439,7 +4429,6 @@ static int wrmsr_interception(struct vcpu_svm *svm)
return 1;
} else {
trace_kvm_msr_write(ecx, data);
- svm->next_rip = kvm_rip_read(&svm->vcpu) + 2;
return kvm_skip_emulated_instruction(&svm->vcpu);
}
}
--
2.20.1