Re: [PATCH v5 4/4] drm/i915: add guard page to ggtt->error_capture

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 08/03/2023 15:39, Andrzej Hajda wrote:
> Write-combining memory allows speculative reads by CPU.
> ggtt->error_capture is WC mapped to CPU, so CPU/MMU can try
> to prefetch memory beyond the error_capture, ie it tries
> to read memory pointed by next PTE in GGTT.
> If this PTE points to invalid address DMAR errors will occur.
> This behaviour was observed on ADL and RPL platforms.
> To avoid it, guard scratch page should be added after error_capture.
> The patch fixes the most annoying issue with error capture but
> since WC reads are used also in other places there is a risk similar
> problem can affect them as well.
> 
> v2:
>    - modified commit message (I hope the diagnosis is correct),
>    - added bug checks to ensure scratch is initialized on gen3 platforms.
>      CI produces strange stacktrace for it suggesting scratch[0] is NULL,
>      to be removed after resolving the issue with gen3 platforms.
> v3:
>    - removed bug checks, replaced with gen check.
> v4:
>    - change code for scratch page insertion to support all platforms,
>    - add info in commit message there could be more similar issues
> v5:
>    - check for nop_clear_range instead of gen8 (Tvrtko),
>    - re-insert scratch pages on resume (Tvrtko)
> 
> Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
> Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_ggtt.c | 35 +++++++++++++++++++++++++++++++----
>   1 file changed, 31 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> index b925da42c7cfc4..8fb700fde85c8f 100644
> --- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
> +++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> @@ -502,6 +502,21 @@ static void cleanup_init_ggtt(struct i915_ggtt *ggtt)
>   	mutex_destroy(&ggtt->error_mutex);
>   }
>   
> +static void
> +ggtt_insert_scratch_pages(struct i915_ggtt *ggtt, u64 offset, u64 length)
> +{
> +	struct i915_address_space *vm = &ggtt->vm;
> +
> +	if (vm->clear_range != nop_clear_range)

Hm I thought usually we would add a prefix for exported stuff, like in 
this case i915_vm_nop_clear_range, however I see intel_gtt.h exports a 
bunch of stuff with no prefixes already so I guess you could continue 
like that by inertia. The conundrum also could have been avoided if you 
left it static (leaving out dpt and mock_gtt patches) but no strong 
opinion from me.

> +		return vm->clear_range(vm, offset, length);
> +
> +	while (length > 0) {
> +		vm->insert_page(vm, px_dma(vm->scratch[0]), offset, I915_CACHE_NONE, 0);
> +		offset += I915_GTT_PAGE_SIZE;
> +		length -= I915_GTT_PAGE_SIZE;
> +	}
> +}
> +
>   static int init_ggtt(struct i915_ggtt *ggtt)
>   {
>   	/*
> @@ -550,8 +565,12 @@ static int init_ggtt(struct i915_ggtt *ggtt)
>   		 * paths, and we trust that 0 will remain reserved. However,
>   		 * the only likely reason for failure to insert is a driver
>   		 * bug, which we expect to cause other failures...
> +		 *
> +		 * Since CPU can perform speculative reads on error capture
> +		 * (write-combining allows it) add scratch page after error
> +		 * capture to avoid DMAR errors.
>   		 */
> -		ggtt->error_capture.size = I915_GTT_PAGE_SIZE;
> +		ggtt->error_capture.size = 2 * I915_GTT_PAGE_SIZE;
>   		ggtt->error_capture.color = I915_COLOR_UNEVICTABLE;
>   		if (drm_mm_reserve_node(&ggtt->vm.mm, &ggtt->error_capture))
>   			drm_mm_insert_node_in_range(&ggtt->vm.mm,
> @@ -561,11 +580,15 @@ static int init_ggtt(struct i915_ggtt *ggtt)
>   						    0, ggtt->mappable_end,
>   						    DRM_MM_INSERT_LOW);
>   	}
> -	if (drm_mm_node_allocated(&ggtt->error_capture))
> +	if (drm_mm_node_allocated(&ggtt->error_capture)) {
> +		u64 start = ggtt->error_capture.start;
> +		u64 size = ggtt->error_capture.size;
> +
> +		ggtt_insert_scratch_pages(ggtt, start, size);
>   		drm_dbg(&ggtt->vm.i915->drm,
>   			"Reserved GGTT:[%llx, %llx] for use by error capture\n",
> -			ggtt->error_capture.start,
> -			ggtt->error_capture.start + ggtt->error_capture.size);
> +			start, start + size);
> +	}
>   
>   	/*
>   	 * The upper portion of the GuC address space has a sizeable hole
> @@ -1256,6 +1279,10 @@ void i915_ggtt_resume(struct i915_ggtt *ggtt)
>   
>   	flush = i915_ggtt_resume_vm(&ggtt->vm);
>   
> +	if (drm_mm_node_allocated(&ggtt->error_capture))
> +		ggtt_insert_scratch_pages(ggtt, ggtt->error_capture.start,
> +					  ggtt->error_capture.size);

Maybe it belongs in i915_ggtt_resume_vm since that one deals with PTEs? 
Looks like it to me, but ack either way.

Regards,

Tvrtko

> +
>   	ggtt->invalidate(ggtt);
>   
>   	if (flush)
>