Re: [PATCH v5 27/27] iommu/arm-smmu-v3-kvm: Enable nesting

[Mostafa Saleh <smostafa@google.com>]

On Fri, Nov 28, 2025 at 01:12:52PM -0400, Jason Gunthorpe wrote:
> On Mon, Nov 17, 2025 at 06:48:14PM +0000, Mostafa Saleh wrote:
> > @@ -441,8 +490,32 @@ static void smmu_reshadow_ste(struct hyp_arm_smmu_v3_device *smmu, u32 sid, bool
> >  		hyp_ste_ptr = smmu_get_ste_ptr(smmu, sid, hyp_ste_base);
> >  	}
> >  
> > -	smmu_copy_from_host(smmu, hyp_ste_ptr->data, host_ste_ptr->data,
> > +	smmu_copy_from_host(smmu, target.data, host_ste_ptr->data,
> >  			    STRTAB_STE_DWORDS << 3);
> > +	/*
> > +	 * Typically, STE update is done as the following
> > +	 * 1- Write last 7 dwords, while STE is invalid
> > +	 * 2- CFGI
> > +	 * 3- Write first dword, making STE valid
> > +	 * 4- CFGI
> > +	 * As the SMMU MUST at least load 64 bits atomically
> > +	 * that gurantees that there is no race between writing
> > +	 * the STE and the CFGI where the SMMU observes parts
> > +	 * of the STE.
> > +	 * In the shadow we update the STE to enable nested translation,
> > +	 * which requires updating first 4 dwords.
> > +	 * That is only done if the STE is valid and not in abort.
> > +	 * Which means it happens at step 4)
> > +	 * So we need to also write the last 7 dwords and send CFGI
> > +	 * before writing the first dword.
> > +	 * There is no need for last CFGI as it's done next.
> > +	 */
> 
> This really should share the main driver logic to do STE writes in the
> right order and try to avoid making it non-valid if not necessary.
> 
> This will not properly support all the real-world kernel flows around
> PASID with such a simplistic implementation.
> 
>

I see, would it be OK to keep it as this simple now and add this later,
as this is still early support? as I want to keep this series minimal.

I plan to have another seires with some improvements and optimizations,
mostly about page tables (using block mappings) and I can include such
optimization, specially that the new hitless STE stuff is relatively
recent in the main driver.

Thanks,
Mostafa

> Json