Re: [PATCH 3/3] hw/arm/smmuv3: Advertise SMMUv3.1-XNX feature

[Mostafa Saleh <smostafa@google.com>]

Hi Peter,

On Thu, Sep 14, 2023 at 03:57:05PM +0100, Peter Maydell wrote:
> The SMMUv3.1-XNX feature is mandatory for an SMMUv3.1 if S2P is
> supported, so we should theoretically have implemented it as part of
> the recent S2P work.  Fortunately, for us the implementation is a
> no-op.
> 
> This feature is about interpretation of the stage 2 page table
> descriptor XN bits, which control execute permissions.
> 
> For QEMU, the permission bits passed to an IOMMU (via MemTxAttrs and
> IOMMUAccessFlags) only indicate read and write; we do not distinguish
> data reads from instruction reads outside the CPU proper.  In the
> SMMU architecture's terms, our interconnect between the client device
> and the SMMU doesn't have the ability to convey the INST attribute,
> and we therefore use the default value of "data" for this attribute.
> 
> We also do not support the bits in the Stream Table Entry that can
> override the on-the-bus transaction attribute permissions (we do not
> set SMMU_IDR1.ATTR_PERMS_OVR=1).
> 
> These two things together mean that for our implementation, it never
> has to deal with transactions with the INST attribute, and so it can
> correctly ignore the XN bits entirely.  So we already implement
> FEAT_XNX's "XN field is now 2 bits, not 1" behaviour to the extent
> that we need to.
> 
> Advertise the presence of the feature in SMMU_IDR3.XNX.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  hw/arm/smmuv3.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/hw/arm/smmuv3.c b/hw/arm/smmuv3.c
> index 94d388fc950..d9e639f7c41 100644
> --- a/hw/arm/smmuv3.c
> +++ b/hw/arm/smmuv3.c
> @@ -279,6 +279,7 @@ static void smmuv3_init_regs(SMMUv3State *s)
>      s->idr[1] = FIELD_DP32(s->idr[1], IDR1, CMDQS,   SMMU_CMDQS);
>  
>      s->idr[3] = FIELD_DP32(s->idr[3], IDR3, HAD, 1);
> +    s->idr[3] = FIELD_DP32(s->idr[3], IDR3, XNX, 1);
May be this can be guarded when S2P is present? according the UM
"In SMMUv3.1 and later, support for this feature is mandatory when
stage 2 is supported, that is when SMMU_IDR0.S2P == 1."
So I am not sure what it would mean for XNX and S1P only.

>      s->idr[3] = FIELD_DP32(s->idr[3], IDR3, RIL, 1);
>      s->idr[3] = FIELD_DP32(s->idr[3], IDR3, BBML, 2);

> -- 
> 2.34.1

Thanks,
Mostafa