Re: [PATCH v19 110/130] KVM: TDX: Handle TDX PV MMIO hypercall

Binbin Wu
2024-07-18T07:33:32Z

On 7/17/2024 6:25 AM, Isaku Yamahata wrote:
> On Tue, Jun 25, 2024 at 02:54:09PM +0800,
> Binbin Wu <binbin.wu@linux.intel.com> wrote:
>
>>
>> On 2/26/2024 4:26 PM, isaku.yamahata@intel.com wrote:
>>> From: Sean Christopherson <sean.j.christopherson@intel.com>
>>>
>>> Export kvm_io_bus_read and kvm_mmio tracepoint and wire up TDX PV MMIO
>>> hypercall to the KVM backend functions.
>>>
>>> kvm_io_bus_read/write() searches KVM device emulated in kernel of the given
>>> MMIO address and emulates the MMIO. As TDX PV MMIO also needs it, export
>>> kvm_io_bus_read(). kvm_io_bus_write() is already exported. TDX PV MMIO
>>> emulates some of MMIO itself. To add trace point consistently with x86
>>> kvm, export kvm_mmio tracepoint.
>>>
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>>> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>>> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
>>> ---
>>> arch/x86/kvm/vmx/tdx.c | 114 +++++++++++++++++++++++++++++++++++++++++
>>> arch/x86/kvm/x86.c | 1 +
>>> virt/kvm/kvm_main.c | 2 +
>>> 3 files changed, 117 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
>>> index 55fc6cc6c816..389bb95d2af0 100644
>>> --- a/arch/x86/kvm/vmx/tdx.c
>>> +++ b/arch/x86/kvm/vmx/tdx.c
>>> @@ -1217,6 +1217,118 @@ static int tdx_emulate_io(struct kvm_vcpu *vcpu)
>>> return ret;
>>> }
>>> +static int tdx_complete_mmio(struct kvm_vcpu *vcpu)
>>> +{
>>> + unsigned long val = 0;
>>> + gpa_t gpa;
>>> + int size;
>>> +
>>> + KVM_BUG_ON(vcpu->mmio_needed != 1, vcpu->kvm);
>>> + vcpu->mmio_needed = 0;
>> mmio_needed is used by instruction emulator to setup the complete callback.
>> Since TDX handle MMIO in a PV way, mmio_needed is not needed here.
> Ok, we don't need to update mmio_needed.
>
>
>>> +
>>> + if (!vcpu->mmio_is_write) {
>> It's also needed by instruction emulator, we can use
>> vcpu->run->mmio.is_write instead.
> No because vcpu->run->mmio is shared with user space. KVM need to stash
> it independently.
>
>
>>> + gpa = vcpu->mmio_fragments[0].gpa;
>>> + size = vcpu->mmio_fragments[0].len;
>> Since MMIO cross page boundary is not allowed according to the input checks
>> from TDVMCALL, these mmio_fragments[] is not needed.
>> Just use vcpu->run->mmio.phys_addr and vcpu->run->mmio.len?
> ditto.
>
>
>>> +
>>> + memcpy(&val, vcpu->run->mmio.data, size);
>>> + tdvmcall_set_return_val(vcpu, val);
>>> + trace_kvm_mmio(KVM_TRACE_MMIO_READ, size, gpa, &val);
>>> + }
>> Tracepoint for KVM_TRACE_MMIO_WRITE is missing when it is handled in
>> userspace.
> tdx_mmio_write() has it before existing to the user space. It matches with
> how write_mmio() behaves in x86.c.
>
> Hmm, to match with other code, we should remove
> trace_kvm_mmio(KVM_TRACE_MMIO_READ) and keep KVM_TRACE_MMIO_READ_UNSATISFIED
> in tdx_emulate_mmio(). That's how read_prepare() and read_exit_mmio() behaves.
>
> For MMIO read
> - When kernel can handle the MMIO, KVM_TRACE_MMIO_READ with data.
> - When exiting to the user space, KVM_TRACE_MMIO_READ_UNSATISFIED before
> the exit. No trace after the user space handled the MMIO.

For MMIO read, in the emulator, there is still a trace after the
userspace handled the MMIO.
In complete_emulated_mmio(), if all fragments have been handled, it will
set vcpu->mmio_read_completed to 1 and call complete_emulated_io().
complete_emulated_io
kvm_emulate_instruction(vcpu, EMULTYPE_NO_DECODE)
x86_emulate_instruction
x86_emulate_insn
emulator_read_write
read_prepare
At this point, vcpu->mmio_read_completed is 1,
it traces KVM_TRACE_MMIO_READ with data
and then clear vcpu->mmio_read_completed

So to align with emulator, we should keep the trace for KVM_TRACE_MMIO_READ.

>
> For MMIO write
> - KVM_TRACE_MMIO_WRITE before handling it.
>
>
>> Also, the return code is only set when the emulation is done in kernel, but
>> not set when it's handled in userspace.
>>
>>> + return 1;
>>> +}
>> How about the fixup as following:
>>
>> @@ -1173,19 +1173,18 @@ static int tdx_emulate_io(struct kvm_vcpu *vcpu)
>> static int tdx_complete_mmio(struct kvm_vcpu *vcpu) { unsigned long val = 0;
>> - gpa_t gpa; - int size; - - vcpu->mmio_needed = 0; - - if
>> (!vcpu->mmio_is_write) { - gpa = vcpu->mmio_fragments[0].gpa; - size =
>> vcpu->mmio_fragments[0].len; + gpa_t gpa = vcpu->run->mmio.phys_addr; + int
>> size = vcpu->run->mmio.len; + if (vcpu->run->mmio.is_write) { +
>> trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, size, gpa, &val); + } else {
>> memcpy(&val, vcpu->run->mmio.data, size); tdvmcall_set_return_val(vcpu,
>> val); trace_kvm_mmio(KVM_TRACE_MMIO_READ, size, gpa, &val); } + +
>> tdvmcall_set_return_code(vcpu, TDVMCALL_SUCCESS); return 1; }
>>
>>
>>
>>> +
>>> +static inline int tdx_mmio_write(struct kvm_vcpu *vcpu, gpa_t gpa, int size,
>>> + unsigned long val)
>>> +{
>>> + if (kvm_iodevice_write(vcpu, &vcpu->arch.apic->dev, gpa, size, &val) &&
>>> + kvm_io_bus_write(vcpu, KVM_MMIO_BUS, gpa, size, &val))
>>> + return -EOPNOTSUPP;
>>> +
>>> + trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, size, gpa, &val);
>>> + return 0;
>>> +}
>>> +
>>> +static inline int tdx_mmio_read(struct kvm_vcpu *vcpu, gpa_t gpa, int size)
>>> +{
>>> + unsigned long val;
>>> +
>>> + if (kvm_iodevice_read(vcpu, &vcpu->arch.apic->dev, gpa, size, &val) &&
>>> + kvm_io_bus_read(vcpu, KVM_MMIO_BUS, gpa, size, &val))
>>> + return -EOPNOTSUPP;
>>> +
>>> + tdvmcall_set_return_val(vcpu, val);
>>> + trace_kvm_mmio(KVM_TRACE_MMIO_READ, size, gpa, &val);
>>> + return 0;
>>> +}
>>> +
>>> +static int tdx_emulate_mmio(struct kvm_vcpu *vcpu)
>>> +{
>>> + struct kvm_memory_slot *slot;
>>> + int size, write, r;
>>> + unsigned long val;
>>> + gpa_t gpa;
>>> +
>>> + KVM_BUG_ON(vcpu->mmio_needed, vcpu->kvm);
>>> +
>> [...]
>>> +
>>> + /* Request the device emulation to userspace device model. */
>>> + vcpu->mmio_needed = 1;
>>> + vcpu->mmio_is_write = write;
>> Then they can be dropped.
> We may drop mmio_needed. mmio_is_write is needed as above.
>
>
>
>>> + vcpu->arch.complete_userspace_io = tdx_complete_mmio;
>>> +
>>> + vcpu->run->mmio.phys_addr = gpa;
>>> + vcpu->run->mmio.len = size;
>>> + vcpu->run->mmio.is_write = write;
>>> + vcpu->run->exit_reason = KVM_EXIT_MMIO;
>>> +
>>> + if (write) {
>>> + memcpy(vcpu->run->mmio.data, &val, size);
>>> + } else {
>>> + vcpu->mmio_fragments[0].gpa = gpa;
>>> + vcpu->mmio_fragments[0].len = size;
>> These two lines can be dropped as well.
> ditto.