Re: [PATCH] KVM: arm64: Fix 64 bit mmio handle

[Marc Zyngier]

On Thu, 30 Jun 2022 17:12:20 +0100,
Schspa Shi <schspa@xxxxxxxxx> wrote:
> 
> If the len is 8 bytes, we can't get the correct sign extend for
> be system.

I'm afraid you'll have to give me a bit more details.

> 
> Fix the mask type len and the comparison of length.
> 
> Signed-off-by: Schspa Shi <schspa@xxxxxxxxx>
> ---
>  arch/arm64/kvm/mmio.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
> index 3dd38a151d2a6..0692f8b18f35c 100644
> --- a/arch/arm64/kvm/mmio.c
> +++ b/arch/arm64/kvm/mmio.c
> @@ -81,8 +81,8 @@ unsigned long kvm_mmio_read_buf(const void *buf, unsigned int len)
>  int kvm_handle_mmio_return(struct kvm_vcpu *vcpu)
>  {
>  	unsigned long data;
> +	unsigned long mask;
>  	unsigned int len;
> -	int mask;
>  
>  	/* Detect an already handled MMIO return */
>  	if (unlikely(!vcpu->mmio_needed))
> @@ -97,7 +97,7 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu)
>  		data = kvm_mmio_read_buf(run->mmio.data, len);
>  
>  		if (kvm_vcpu_dabt_issext(vcpu) &&
> -		    len < sizeof(unsigned long)) {
> +		    len <= sizeof(unsigned long)) {

If you're reading an 8 byte quantity, what is there to sign-extend?
Sign extension only makes sense if what you're reading is *smaller*
than the size of the register you are targeting.

I must be missing something. And how is that related to running BE? BE
in the host? The guest?

Please convince me.

	M.

-- 
Without deviation from the norm, progress is not possible.