Re: [PATCH rc] iommupt: Only cache flush memory changed by unmap

[Francois Dugast <francois.dugast@intel.com>]

On Sat, Jan 24, 2026 at 05:00:21PM -0400, Jason Gunthorpe wrote:
> The cache flush was happening on every level across the whole range of
> iteration, even if no leafs or tables were cleared. Instead flush only the
> sub range that was actually written.
> 
> Overflushing isn't a correctness problem but it does impact the
> performance of unmap.
> 
> After this series the performance compared to the original VT-d
> implementation with cache flushing turned on is:
> 
> map_pages
>    pgsz  ,avg new,old ns, min new,old ns  , min % (+ve is better)
>      2^12,    253,266   ,     213,227     ,   6.06
>      2^21,    246,244   ,     221,219     ,   0.00
>      2^30,    231,240   ,     209,217     ,   3.03
>  256*2^12,   2604,2668  ,    2415,2540    ,   4.04
>  256*2^21,   2495,2824  ,    2390,2734    ,  12.12
>  256*2^30,   2542,2845  ,    2380,2718    ,  12.12
> 
> unmap_pages
>    pgsz  ,avg new,old ns, min new,old ns  , min % (+ve is better)
>      2^12,    259,292   ,     222,251     ,  11.11
>      2^21,    255,259   ,     227,236     ,   3.03
>      2^30,    238,254   ,     217,230     ,   5.05
>  256*2^12,   2751,2620  ,    2417,2437    ,   0.00
>  256*2^21,   2461,2526  ,    2377,2423    ,   1.01
>  256*2^30,   2498,2543  ,    2370,2404    ,   1.01
> 
> Fixes: efa03dab7ce4 ("iommupt: Flush the CPU cache after any writes to the page table")
> Reported-by: Francois Dugast <francois.dugast@intel.com>
> Closes: https://lore.kernel.org/all/20260121130233.257428-1-francois.dugast@intel.com/
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

This reverts the performance drop I reported, thanks.

Tested-by: Francois Dugast <francois.dugast@intel.com>

Francois