Re: [Intel-gfx] [PATCH v4] drm/i915: add guard page to ggtt->error_capture

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

On 08/02/2023 10:51, 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, RPL, DG2 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.
> 
> Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
> Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>

Research tells the explanation is plausible so:

Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

On patch details below.

What about "simiar risk in other places" - are there any plans to asses 
the potential impact elsewhere?

> ---
> This patch tries to diminish plague of DMAR read errors present
> in CI for ADL*, RPL*, DG2 platforms, see for example [1] (grep DMAR).
> CI is usually tolerant for these errors, so the scale of the problem
> is not really visible.
> To show it I have counted lines containing DMAR read errors in dmesgs
> produced by CI for all three versions of the patch, but in contrast to v2
> I have grepped only for lines containing "PTE Read access".
> Below stats for kernel w/o patch vs patched one.
> v1: 210 vs 0
> v2: 201 vs 0
> v3: 214 vs 0
> Apparently the patch fixes all common PTE read errors.
> 
> In previous version there were different numbers due to less exact grepping,
> "grep DMAR" catched write errors and "DMAR: DRHD: handling fault status reg"
> lines, anyway the actual number of errors is much bigger - DMAR errors
> are rate-limited.
> 
> [1]: http://gfx-ci.igk.intel.com/tree/drm-tip/CI_DRM_12678/bat-adln-1/dmesg0.txt
> 
> Changelog:
> 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
> 
> Regards
> Andrzej
> ---
>   drivers/gpu/drm/i915/gt/intel_ggtt.c | 31 ++++++++++++++++++++++++----
>   1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> index 842e69c7b21e49..6566d2066f1f8b 100644
> --- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
> +++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> @@ -503,6 +503,21 @@ 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 (GRAPHICS_VER(ggtt->vm.i915) < 8)
> +		return vm->clear_range(vm, offset, length);
> +	/* clear_range since gen8 is nop */

It would also work to simply drop the <gen8 case and just do the loop 
below, right? If so would that be clearer?

> +	while (length > 0) {

Maybe add a GEM_BUG_ON if length is not aligned? Granted it may be a 
huge stretch of imagination..

Regards,

Tvrtko

> +		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)
>   {
>   	/*
> @@ -551,8 +566,12 @@ 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,
> @@ -562,11 +581,15 @@ 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