Re: [PATCH 1/2] KVM: arm64: Fix sign-extension of MMIO loads

[Marc Zyngier]

On Mon, 22 Jun 2026 20:07:00 +0100,
Fuad Tabba <fuad.tabba@xxxxxxxxx> wrote:
> 
> A sign-extending load from MMIO (LDRSB, LDRSH, LDRSW) delivers a
> zero-extended value: in kvm_handle_mmio_return(), vcpu_data_host_to_guest()
> masks the data to the access width after sign-extension, stripping the
> sign bits.
> 
> Move vcpu_data_host_to_guest() ahead of sign-extension so the width mask
> runs first. trace_kvm_mmio() moves with it and keeps reporting the raw
> access-width data.
> 
> Fixes: b30070862edbd ("ARM64: KVM: MMIO support BE host running LE code")
> Signed-off-by: Fuad Tabba <fuad.tabba@xxxxxxxxx>
> ---
>  arch/arm64/kvm/mmio.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
> index e2285ed8c91de..d1c3a352d5a22 100644
> --- a/arch/arm64/kvm/mmio.c
> +++ b/arch/arm64/kvm/mmio.c
> @@ -126,6 +126,10 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu)
>  		len = kvm_vcpu_dabt_get_as(vcpu);
>  		data = kvm_mmio_read_buf(run->mmio.data, len);
>  
> +		trace_kvm_mmio(KVM_TRACE_MMIO_READ, len, run->mmio.phys_addr,
> +			       &data);
> +		data = vcpu_data_host_to_guest(vcpu, data, len);

This helper is in charge of the endianness of the read from the
guest's PoV. How can the sign-extension (which happens from the host's
perspective) correctly work if it takes place after the byte swapping?

To me, the real issue appears to be in the that helper, which swaps
the data based on the size of the access instead of the width of the
register.

Or am I once more completely confused with the endianness crap?

	M.

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