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

[Paolo Bonzini <pbonzini@redhat.com>]

On 03/03/2015 21:42, Radim Krčmář wrote:
> 2015-03-03 13:48-0600, Joel Schopp:
>>>> +	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?
> 
> According to APM vol.2, 2.5.3 Operands and Results, when using EAX,
> we should zero upper 32 bits of RAX:
> 
>   Zero Extension of Results. In 64-bit mode, when performing 32-bit
>   operations with a GPR destination, the processor zero-extends the 32-bit
>   result into the full 64-bit destination. Both 8-bit and 16-bit
>   operations on GPRs preserve all unwritten upper bits of the destination
>   GPR. This is consistent with legacy 16-bit and 32-bit semantics for
>   partial-width results.
> 
> Is IN not covered?

It is.  You need to zero the upper 32 bits.

>>>> +	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.
> 
> BUG_ON is checked on essentially all kernels that run KVM.
> (All distribution-based configs should have it.)

Correct.

> If we wanted to validate the size, then this is strictly better:
>   BUG_ON(vcpu->arch.pio.count != 1 || vcpu->arch.pio.size > sizeof(new_rax))

That would be a very weird assertion considering that
vcpu->arch.pio.size will architecturally be at most 4.

The first arm of the || is sufficient.

>>>> +	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.
> 
> Pasting the same code creates bug opportunities when we forget to modify
> all places.  This class of problems can be harder to deal with, that (c)
> and (d), because we can't simply print all callers.

I agree with this and prefer calling emulator_pio_in_emulated in
complete_fast_pio_in, indeed.

>>> Refactoring could avoid the weird vcpu->ctxt->vcpu conversion.
>>> (A better name is always welcome.)

No need for that.

>> 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?

Because the emulator is written to be usable outside the Linux kernel as
well.

Also, the fast path (used if kernel_pio returns 0) doesn't read
VCPU_REGS_RAX, thus using an uninitialized variable here:

>>> +	unsigned long val;
>>> +	int ret = emulator_pio_in_emulated(&vcpu->arch.emulate_ctxt, size,
>>> +					   port, &val, 1);
>>> +
>>> +	if (ret)
>>> +		kvm_register_write(vcpu, VCPU_REGS_RAX, val);

Thanks,

Paolo