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SubjectRe: [PATCH 2/2] KVM: x86: On emulation failure, convey the exit reason to userspace
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On Wednesday, 2021-06-30 at 16:48:42 UTC, David Matlack wrote:

> On Mon, Jun 28, 2021 at 06:31:52PM +0100, David Edmondson wrote:
>> To aid in debugging.
>
> Please add more context to the commit message.

Okay.

>>
>> Suggested-by: Joao Martins <joao.m.martins@oracle.com>
>> Signed-off-by: David Edmondson <david.edmondson@oracle.com>
>> ---
>> arch/x86/kvm/x86.c | 23 +++++++++++++++++------
>> include/uapi/linux/kvm.h | 2 ++
>> 2 files changed, 19 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 8166ad113fb2..48ef0dc68faf 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -7455,7 +7455,7 @@ void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip)
>> }
>> EXPORT_SYMBOL_GPL(kvm_inject_realmode_interrupt);
>>
>> -static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
>> +static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu, uint64_t flags)
>> {
>> struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
>> u32 insn_size = ctxt->fetch.end - ctxt->fetch.data;
>> @@ -7466,7 +7466,8 @@ static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
>> run->emulation_failure.ndata = 0;
>> run->emulation_failure.flags = 0;
>>
>> - if (insn_size) {
>> + if (insn_size &&
>> + (flags & KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES)) {
>> run->emulation_failure.ndata = 3;
>> run->emulation_failure.flags |=
>> KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES;
>> @@ -7476,6 +7477,14 @@ static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
>> memcpy(run->emulation_failure.insn_bytes,
>> ctxt->fetch.data, insn_size);
>> }
>> +
>> + if (flags & KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON) {
>
> This flag is always passed so this check if superfluous. Perhaps change
> `int flags` to `bool instruction_bytes` and have it control only whether
> the instruction bytes are populated.

Okay.

>> + run->emulation_failure.ndata = 4;
>> + run->emulation_failure.flags |=
>> + KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON;
>> + run->emulation_failure.exit_reason =
>> + static_call(kvm_x86_get_exit_reason)(vcpu);
>> + }
>> }
>>
>> static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
>> @@ -7492,16 +7501,18 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
>>
>> if (kvm->arch.exit_on_emulation_error ||
>> (emulation_type & EMULTYPE_SKIP)) {
>> - prepare_emulation_failure_exit(vcpu);
>> + prepare_emulation_failure_exit(
>> + vcpu,
>> + KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES |
>> + KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON);
>> return 0;
>> }
>>
>> kvm_queue_exception(vcpu, UD_VECTOR);
>>
>> if (!is_guest_mode(vcpu) && static_call(kvm_x86_get_cpl)(vcpu) == 0) {
>> - vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
>> - vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
>> - vcpu->run->internal.ndata = 0;
>> + prepare_emulation_failure_exit(
>> + vcpu, KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON);
>
> Should kvm_task_switch and kvm_handle_memory_failure also be updated
> like this?

Will do in v2.

sgx_handle_emulation_failure() seems like an existing user of
KVM_INTERNAL_ERROR_EMULATION that doesn't follow the new protocol (use
the emulation_failure part of the union).

Sean: If I add another flag for this case, what is the existing
user-level consumer?

>> return 0;
>> }
>>
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 68c9e6d8bbda..3e4126652a67 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -282,6 +282,7 @@ struct kvm_xen_exit {
>>
>> /* Flags that describe what fields in emulation_failure hold valid data. */
>> #define KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES (1ULL << 0)
>> +#define KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON (1ULL << 1)
>>
>> /* for KVM_RUN, returned by mmap(vcpu_fd, offset=0) */
>> struct kvm_run {
>> @@ -404,6 +405,7 @@ struct kvm_run {
>> __u64 flags;
>> __u8 insn_size;
>> __u8 insn_bytes[15];
>> + __u64 exit_reason;
>
> Please document what this field contains, especially since its contents
> depend on AMD versus Intel.

Okay.

>> } emulation_failure;
>> /* KVM_EXIT_OSI */
>> struct {
>> --
>> 2.30.2
>>

dme.
--
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