Re: [PATCH v16 1/5] arm/vpci: honor access size when returning an error

[Stewart Hildebrand <stewart.hildebrand@amd.com>]

On 5/22/24 18:59, Stewart Hildebrand wrote:
> From: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
> 
> Guest can try to read config space using different access sizes: 8,
> 16, 32, 64 bits. We need to take this into account when we are
> returning an error back to MMIO handler, otherwise it is possible to
> provide more data than requested: i.e. guest issues LDRB instruction
> to read one byte, but we are writing 0xFFFFFFFFFFFFFFFF in the target
> register.
> 
> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
> Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>

I forgot to pick up Julien's ack [0].

[0] https://lore.kernel.org/xen-devel/8fa02e06-d8dc-4e73-a58e-e4d84b090ea8@xen.org/

> ---
> v14->v15:
> * re-order so this patch comes before ("xen/arm: translate virtual PCI
>   bus topology for guests")
> * s/access_mask/invalid/
> * add U suffix to 1
> * s/uint8_t/unsigned int/
> * s/uint64_t/register_t/
> * although Julien gave an Acked-by on v14, I omitted it due to the
>   changes made in v15
> 
> v9->10:
> * New patch in v10.
> ---
>  xen/arch/arm/vpci.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/vpci.c b/xen/arch/arm/vpci.c
> index 3bc4bb55082a..b63a356bb4a8 100644
> --- a/xen/arch/arm/vpci.c
> +++ b/xen/arch/arm/vpci.c
> @@ -29,6 +29,8 @@ static int vpci_mmio_read(struct vcpu *v, mmio_info_t *info,
>  {
>      struct pci_host_bridge *bridge = p;
>      pci_sbdf_t sbdf = vpci_sbdf_from_gpa(bridge, info->gpa);
> +    const unsigned int access_size = (1U << info->dabt.size) * 8;
> +    const register_t invalid = GENMASK_ULL(access_size - 1, 0);
>      /* data is needed to prevent a pointer cast on 32bit */
>      unsigned long data;
>  
> @@ -39,7 +41,7 @@ static int vpci_mmio_read(struct vcpu *v, mmio_info_t *info,
>          return 1;
>      }
>  
> -    *r = ~0ul;
> +    *r = invalid;
>  
>      return 0;
>  }