Re: [Xen-devel] [PATCH for 4.6] xen/arm: vgic: Correctly emulate write when byte is used

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Sep 22, 2015 at 09:18:48PM +0100, Julien Grall wrote:
> When a guest is writing a byte, the value will be located in bits[7:0]
> of the register.
> 
> Although the current implementation is expecting the byte at the Nth
> byte of the register where N = address & 4;
> 
> When the address is not 4-byte aligned, the corresponding byte in the
> internal state will always be set to zero rather.
> 
> Note that byte access are only used for GICD_IPRIORITYR and
> GICD_ITARGETSR. So the worst things that could happen is not setting the
> priority correctly and ignore the target vCPU written.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> 

Subject to an ack / review from ARM folks:

Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

> ---
> 
> This patch is a candidate for Xen 4.6 and backport to Xen 4.5 and Xen
> 4.4. Without it, write a byte to a register won't work as expected.
> 
> Spotted while doing some test on the vGICv2 driver. FWIW, Linux doesn't
> use byte access in both GICv3 and GICv2 driver.
> 
> Note that Xen 4.4 will require a different patch because the function
> was living in vgic.c at that time.
> ---
>  xen/include/asm-arm/vgic.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/include/asm-arm/vgic.h b/xen/include/asm-arm/vgic.h
> index 41cadb1..96839f0 100644
> --- a/xen/include/asm-arm/vgic.h
> +++ b/xen/include/asm-arm/vgic.h
> @@ -174,10 +174,10 @@ static inline void vgic_byte_write(uint32_t *reg, 
> uint32_t var, int offset)
>  {
>      int byte = offset & 0x3;
>  
> -    var &= (0xff << (8*byte));
> +    var &= 0xff;
>  
>      *reg &= ~(0xff << (8*byte));
> -    *reg |= var;
> +    *reg |= (var << (8*byte));
>  }
>  
>  enum gic_sgi_mode;
> -- 
> 2.1.4

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