Re: [PATCH v2] iommu/vt-d: calculate mask for non-aligned flushes

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi David,

On 2022/3/22 14:35, David Stevens wrote:
> From: David Stevens <stevensd@chromium.org>
> 
> Calculate the appropriate mask for non-size-aligned page selective
> invalidation. Since psi uses the mask value to mask out the lower order
> bits of the target address, properly flushing the iotlb requires using a
> mask value such that [pfn, pfn+pages) all lie within the flushed
> size-aligned region.  This is not normally an issue because iova.c
> always allocates iovas that are aligned to their size. However, iovas
> which come from other sources (e.g. userspace via VFIO) may not be
> aligned.

This is bug fix, right? Can you please add "Fixes" and "Cc stable" tags?

> 
> Signed-off-by: David Stevens <stevensd@chromium.org>
> ---
> v1 -> v2:
>   - Calculate an appropriate mask for non-size-aligned iovas instead
>     of falling back to domain selective flush.
> 
>   drivers/iommu/intel/iommu.c | 27 ++++++++++++++++++++++++---
>   1 file changed, 24 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 5b196cfe9ed2..ab2273300346 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -1717,7 +1717,8 @@ static void iommu_flush_iotlb_psi(struct intel_iommu *iommu,
>   				  unsigned long pfn, unsigned int pages,
>   				  int ih, int map)
>   {
> -	unsigned int mask = ilog2(__roundup_pow_of_two(pages));
> +	unsigned int aligned_pages = __roundup_pow_of_two(pages);
> +	unsigned int mask = ilog2(aligned_pages);
>   	uint64_t addr = (uint64_t)pfn << VTD_PAGE_SHIFT;
>   	u16 did = domain->iommu_did[iommu->seq_id];
>   
> @@ -1729,10 +1730,30 @@ static void iommu_flush_iotlb_psi(struct intel_iommu *iommu,
>   	if (domain_use_first_level(domain)) {
>   		domain_flush_piotlb(iommu, domain, addr, pages, ih);
>   	} else {
> +		unsigned long bitmask = aligned_pages - 1;
> +
> +		/*
> +		 * PSI masks the low order bits of the base address. If the
> +		 * address isn't aligned to the mask, then compute a mask value
> +		 * needed to ensure the target range is flushed.
> +		 */
> +		if (unlikely(bitmask & pfn)) {
> +			unsigned long end_pfn = pfn + pages - 1, shared_bits;
> +
> +			/*
> +			 * Since end_pfn <= pfn + bitmask, the only way bits
> +			 * higher than bitmask can differ in pfn and end_pfn is
> +			 * by carrying. This means after masking out bitmask,
> +			 * high bits starting with the first set bit in
> +			 * shared_bits are all equal in both pfn and end_pfn.
> +			 */
> +			shared_bits = ~(pfn ^ end_pfn) & ~bitmask;
> +			mask = shared_bits ? __ffs(shared_bits) : BITS_PER_LONG;

Can you please add some lines in the commit message to explain how this
magic line works? It's easier for people to understand it if you can
take a real example. :-)

Best regards,
baolu

> +		}
> +
>   		/*
>   		 * Fallback to domain selective flush if no PSI support or
> -		 * the size is too big. PSI requires page size to be 2 ^ x,
> -		 * and the base address is naturally aligned to the size.
> +		 * the size is too big.
>   		 */
>   		if (!cap_pgsel_inv(iommu->cap) ||
>   		    mask > cap_max_amask_val(iommu->cap))