Re: [PATCH v2] arm64: KVM: Only skip MMIO insn once

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Aug 22, 2019 at 01:03:05PM +0200, Andrew Jones wrote:
> If after an MMIO exit to userspace a VCPU is immediately run with an
> immediate_exit request, such as when a signal is delivered or an MMIO
> emulation completion is needed, then the VCPU completes the MMIO
> emulation and immediately returns to userspace. As the exit_reason
> does not get changed from KVM_EXIT_MMIO in these cases we have to
> be careful not to complete the MMIO emulation again, when the VCPU is
> eventually run again, because the emulation does an instruction skip
> (and doing too many skips would be a waste of guest code :-) We need
> to use additional VCPU state to track if the emulation is complete.
> As luck would have it, we already have 'mmio_needed', which even
> appears to be used in this way by other architectures already.
> 
> Fixes: 0d640732dbeb ("arm64: KVM: Skip MMIO insn after emulation")
> Signed-off-by: Andrew Jones <drjones@redhat.com>

Ouch; sorry about this!

I haven't dug too deeply, but from the commit message, the below makes
sense to me. FWIW:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
> v2: move mmio_needed use closer to other mmio state use [maz]
> 
>  virt/kvm/arm/mmio.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
> index a8a6a0c883f1..6af5c91337f2 100644
> --- a/virt/kvm/arm/mmio.c
> +++ b/virt/kvm/arm/mmio.c
> @@ -86,6 +86,12 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  	unsigned int len;
>  	int mask;
>  
> +	/* Detect an already handled MMIO return */
> +	if (unlikely(!vcpu->mmio_needed))
> +		return 0;
> +
> +	vcpu->mmio_needed = 0;
> +
>  	if (!run->mmio.is_write) {
>  		len = run->mmio.len;
>  		if (len > sizeof(unsigned long))
> @@ -188,6 +194,7 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
>  	run->mmio.is_write	= is_write;
>  	run->mmio.phys_addr	= fault_ipa;
>  	run->mmio.len		= len;
> +	vcpu->mmio_needed	= 1;
>  
>  	if (!ret) {
>  		/* We handled the access successfully in the kernel. */
> -- 
> 2.18.1
>