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Re: [Xen-devel] [RFC v2 7/7] xen/iommu: smmu-v3: Add Xen specific code to enable the ported driver



Hi Sameer,

On 22/11/17 02:17, Goel, Sameer wrote:
On 11/20/2017 7:25 AM, Julien Grall wrote:
On 19/11/17 07:45, Goel, Sameer wrote:
On 10/12/2017 10:36 AM, Julien Grall wrote:
Please use #if 0 rather than removing the code + comment on top. But I am not 
sure why you drop the S2 free code. Shouldn't we allocate VMID from the SMMU?
I am picking up the vmid from the domain I am sharing the page tables with. 
domain->arch.p2m.vmid. This seemed valid. Please let me know if we want to 
generate a different vmid?

The processor and the SMMU may support either 8 or 16bits VMID but I don't 
think any thing in the spec says that both processor and SMMU must support the 
same value.

So it would be possible to have a processor supporting 16-bits VMID and the 
SMMU only 8-bits VMID.

Adding a check at the SMMU initialization might be a solution. But as the code 
for allocating the VMID is already present, then I would prefer to we stick 
with different the VMID.
Sure. For this iteration I will generate an SMMU specific VMID. Do you think it 
would be an optimization to reuse the CPU VMID if SMMU supports 16bit VMID?

The only benefit I can see is if the SMMU support broadcast TLB maintenance. AFAICT, the driver would need to configure the SMMU for that.

However, I think we should first focus on getting a basic SMMUv3 driver in Xen. Once this is done, we can look at optimizing the driver.

Cheers,

--
Julien Grall

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