Re: [PATCH v3] x86: svm: use kvm_fast_pio_in()

Joel Schopp
2015-03-03T19:48:57Z

Thank you for your detailed review on several of my patches.

>>
>> +static int complete_fast_pio(struct kvm_vcpu *vcpu)
> (complete_fast_pio_in()?)
If I do a v4 I'll adopt that name.
>> +{
>> + unsigned long new_rax = kvm_register_read(vcpu, VCPU_REGS_RAX);
> Shouldn't we handle writes in EAX differently than in AX and AL, because
> of implicit zero extension.
I don't think the implicit zero extension hurts us here, but maybe there
is something I'm missing that I need understand. Could you explain this
further?
>
>> +
>> + BUG_ON(!vcpu->arch.pio.count);
>> + BUG_ON(vcpu->arch.pio.count * vcpu->arch.pio.size > sizeof(new_rax));
> (Looking at it again, a check for 'vcpu->arch.pio.count == 1' would be
> sufficient.)
I prefer the checks that are there now after your last review,
especially since surrounded by BUG_ON they only run on debug kernels.

>
>> +
>> + memcpy(&new_rax, vcpu, sizeof(new_rax));
>> + trace_kvm_pio(KVM_PIO_IN, vcpu->arch.pio.port, vcpu->arch.pio.size,
>> + vcpu->arch.pio.count, vcpu->arch.pio_data);
>> + kvm_register_write(vcpu, VCPU_REGS_RAX, new_rax);
>> + vcpu->arch.pio.count = 0;
> I think it is better to call emulator_pio_in_emulated directly, like
>
> emulator_pio_in_out(&vcpu->arch.emulate_ctxt, vcpu->arch.pio.size,
> vcpu->arch.pio.port, &new_rax, 1);
> kvm_register_write(vcpu, VCPU_REGS_RAX, new_rax);
>
> because we know that vcpu->arch.pio.count != 0.
I think two extra lines of code in my patch vs your suggestion are worth
it to a) reduce execution path length b) increase readability c) avoid
breaking the abstraction by not checking the return code d) avoid any
future bugs introduced by changes the function that would return a value
other than 1.
>
> Refactoring could avoid the weird vcpu->ctxt->vcpu conversion.
> (A better name is always welcome.)
The pointer chasing is making me dizzy. I'm not sure why
emulator_pio_in_emulated takes a x86_emulate_ctxt when all it does it
immediately translate that to a vcpu and never use the x86_emulate_ctxt,
why not pass the vcpu in the first place?