Re: [PATCH v5 1/3] drm/xe/migrate: Atomicize CCS copy command setup

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Thu, Oct 09, 2025 at 09:11:13AM -0700, Matthew Brost wrote:
> On Thu, Oct 09, 2025 at 09:00:43AM -0400, Rodrigo Vivi wrote:
> > On Wed, Oct 08, 2025 at 03:58:32PM -0700, Matthew Brost wrote:
> > > On Wed, Oct 08, 2025 at 03:41:47PM +0530, Satyanarayana K V P wrote:
> > > > The CCS copy command is a 5-dword sequence. If the vCPU halts during
> > > > save/restore while this sequence is being programmed, partial writes may
> > > > trigger page faults when saving IGPU CCS metadata. Use the VMOVDQU
> > > > instruction to write the sequence atomically.
> > > > 
> > > > Since VMOVDQU operates on 256-bit chunks, update EMIT_COPY_CCS_DW to emit
> > > > 8 dwords instead of 5 dwords.
> > > > 
> > > > Update emit_flush_invalidate() to use VMOVDQU operating with 128-bit
> > > > chunks.
> > > > 
> > > > Signed-off-by: Satyanarayana K V P <satyanarayana.k.v.p@intel.com>
> > > > Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
> > > > Cc: Matthew Brost <matthew.brost@intel.com>
> > > > Cc: Matthew Auld <matthew.auld@intel.com>
> > > > 
> > > > ---
> > > > V4 -> V5:
> > > > - Fixed review comments. (Matt B)
> > > > 
> > > > V3 -> V4:
> > > > - Fixed review comments. (Wajdeczko)
> > > > - Fix issues reported by patchworks.
> > > > 
> > > > V2 -> V3:
> > > > - Added support for 128 bit and 256 bit instructions with memcpy_vmovdqu
> > > > - Updated emit_flush_invalidate() to use vmovdqu instruction.
> > > > 
> > > > V1 -> V2:
> > > > - Use memcpy_vmovdqu only for x86 arch and for VF. Else use memcpy
> > > >   (Auld, Matthew)
> > > > - Fix issues reported by patchworks.
> > > > ---
> > > >  drivers/gpu/drm/xe/xe_migrate.c | 93 +++++++++++++++++++++++++--------
> > > >  1 file changed, 72 insertions(+), 21 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/xe/xe_migrate.c b/drivers/gpu/drm/xe/xe_migrate.c
> > > > index c39c3b423d05..b629072956ee 100644
> > > > --- a/drivers/gpu/drm/xe/xe_migrate.c
> > > > +++ b/drivers/gpu/drm/xe/xe_migrate.c
> > > > @@ -5,7 +5,9 @@
> > > >  
> > > >  #include "xe_migrate.h"
> > > >  
> > > > +#include <asm/fpu/api.h>
> > > >  #include <linux/bitfield.h>
> > > > +#include <linux/cpufeature.h>
> > > >  #include <linux/sizes.h>
> > > >  
> > > >  #include <drm/drm_managed.h>
> > > > @@ -33,6 +35,7 @@
> > > >  #include "xe_res_cursor.h"
> > > >  #include "xe_sa.h"
> > > >  #include "xe_sched_job.h"
> > > > +#include "xe_sriov_vf_ccs.h"
> > > >  #include "xe_sync.h"
> > > >  #include "xe_trace_bo.h"
> > > >  #include "xe_validation.h"
> > > > @@ -644,18 +647,49 @@ static void emit_pte(struct xe_migrate *m,
> > > >  	}
> > > >  }
> > > >  
> > > > -#define EMIT_COPY_CCS_DW 5
> > > > +static void memcpy_vmovdqu(void *dst, const void *src, u32 size)
> > > > +{
> > > > +#ifdef CONFIG_X86
> > > > +	kernel_fpu_begin();
> > > > +	if (size == SZ_128) {
> > > > +		asm("vmovdqu (%0), %%xmm0\n"
> > > > +		    "vmovups %%xmm0,   (%1)\n"
> > > > +		    :: "r" (src), "r" (dst) : "memory");
> > > > +	} else if (size == SZ_256) {
> > > > +		asm("vmovdqu (%0), %%ymm0\n"
> > > > +		    "vmovups %%ymm0,   (%1)\n"
> > > > +		    :: "r" (src), "r" (dst) : "memory");
> > > > +	}
> > > > +	kernel_fpu_end();
> > > > +#endif
> > > 
> > > Everything in this patch LGTM but I think we maintainer input to ensure
> > > we are breaking some rules about inlined asm code in a driver (no idea
> > > if this exists) or if a better place would be somewhere common. Can you
> > > ping Lucas, Thomas, or Rodrigo and ask them about this?
> > 
> > Well, it is possible and we have asm code in i915 for instance (i915_memcpy.c)
> > 
> > But the rule does exist:
> > https://www.kernel.org/doc/html/latest/process/coding-style.html#inline-assembly
> > 
> > "don’t use inline assembly gratuitously when C can do the job. You can and should
> > poke hardware from C when possible"
> > 
> > In this case here, please explain why exactly memcpy with smp_wmb barriers and
> > or WRITE_ONCE code combined couldn't solve it.
> > 
> > Also, please explain how exactly vmovdqu guarantees the atomicity promised by
> > the commit message. On a quick search here my take is that for this 128 or 256
> > bits, atomicity is not guaranteed.
> 
> I don't think cache atomicity is what we're after here—rather, it's vCPU
> halting atomicity.
> 
> Consider the following case:
> b++ = XY_CTRL_SURF_COPY_BLT;
> b++ = addr;
> 
> If the vCPU is halted during the instruction that stores
> XY_CTRL_SURF_COPY_BLT, the address will be invalid. The GuC executes the
> batch buffer (BB) that is being programmed as part of the VF save. This
> will clearly cause the BB to hang due to a page fault on the copy
> command.

okay, perhaps this is what is getting me confused most
what I don't understand in the flow is: why GuC is already
executing it or going to execute it while you are going to a halt when
writing the command to the buffer?  and not writing to the buffer first
and then sending it to the exec queue?

> 
> If the entire XY_CTRL_SURF_COPY_BLT is stored via an AVX instruction,
> then either the GPU entire instruction is written or none of it is. I
> believe vCPU halting guarantees that a CPU instruction is either fully
> executed or not at all—regardless of how many micro-operations (uOPs) it
> decodes into. If this guarantee does not hold, then the entire
> architecture of CCS save/restore on PTL is fundamentally broken which is
> always possible.

Okay, this is guaranteed. I mean, the vCPU won't get halted in the middle
of the vmovdqu nor vmovups. only before, between, or after them.

But is this uncached and/or coherent? isn't there really any possibility that
the command finished, but GuC mid-flight executing things aren't still
seeing different cachelines?

> 
> > 
> > So, imho this patch is introducing a unmaintainable, complex, and fragile code
> > that is not even doing what it is claiming to do. But I will be glad if someone
> > can challenge this and prove me wrong.
> > 
> 
> Let me know if the above makes any sense.

Okay. But how to handle cases where AVX might not be available? really not needed?

> 
> Matt
> 
> > Thanks,
> > Rodrigo.
> > 
> > > 
> > > Matt 
> > > 
> > > > +}
> > > > +
> > > > +static void emit_atomic(struct xe_gt *gt, void *dst, const void *src, u32 size)
> > > > +{
> > > > +	u32 instr_size = size * BITS_PER_BYTE;
> > > > +
> > > > +	xe_gt_assert(gt, instr_size == SZ_128 || instr_size == SZ_256);
> > > > +
> > > > +	if (IS_VF_CCS_READY(gt_to_xe(gt)) && static_cpu_has(X86_FEATURE_AVX))
> > > > +		memcpy_vmovdqu(dst, src, instr_size);
> > > > +	else
> > > > +		memcpy(dst, src, size);
> > > > +}
> > > > +
> > > > +#define EMIT_COPY_CCS_DW 8
> > > >  static void emit_copy_ccs(struct xe_gt *gt, struct xe_bb *bb,
> > > >  			  u64 dst_ofs, bool dst_is_indirect,
> > > >  			  u64 src_ofs, bool src_is_indirect,
> > > >  			  u32 size)
> > > >  {
> > > > +	u32 dw[EMIT_COPY_CCS_DW] = {MI_NOOP};
> > > >  	struct xe_device *xe = gt_to_xe(gt);
> > > >  	u32 *cs = bb->cs + bb->len;
> > > >  	u32 num_ccs_blks;
> > > >  	u32 num_pages;
> > > >  	u32 ccs_copy_size;
> > > >  	u32 mocs;
> > > > +	u32 i = 0;
> > > >  
> > > >  	if (GRAPHICS_VERx100(xe) >= 2000) {
> > > >  		num_pages = DIV_ROUND_UP(size, XE_PAGE_SIZE);
> > > > @@ -673,15 +707,23 @@ static void emit_copy_ccs(struct xe_gt *gt, struct xe_bb *bb,
> > > >  		mocs = FIELD_PREP(XY_CTRL_SURF_MOCS_MASK, gt->mocs.uc_index);
> > > >  	}
> > > >  
> > > > -	*cs++ = XY_CTRL_SURF_COPY_BLT |
> > > > -		(src_is_indirect ? 0x0 : 0x1) << SRC_ACCESS_TYPE_SHIFT |
> > > > -		(dst_is_indirect ? 0x0 : 0x1) << DST_ACCESS_TYPE_SHIFT |
> > > > -		ccs_copy_size;
> > > > -	*cs++ = lower_32_bits(src_ofs);
> > > > -	*cs++ = upper_32_bits(src_ofs) | mocs;
> > > > -	*cs++ = lower_32_bits(dst_ofs);
> > > > -	*cs++ = upper_32_bits(dst_ofs) | mocs;
> > > > +	dw[i++] = XY_CTRL_SURF_COPY_BLT |
> > > > +		  (src_is_indirect ? 0x0 : 0x1) << SRC_ACCESS_TYPE_SHIFT |
> > > > +		  (dst_is_indirect ? 0x0 : 0x1) << DST_ACCESS_TYPE_SHIFT |
> > > > +		  ccs_copy_size;
> > > > +	dw[i++] = lower_32_bits(src_ofs);
> > > > +	dw[i++] = upper_32_bits(src_ofs) | mocs;
> > > > +	dw[i++] = lower_32_bits(dst_ofs);
> > > > +	dw[i++] = upper_32_bits(dst_ofs) | mocs;
> > > >  
> > > > +	/*
> > > > +	 * The CCS copy command is a 5-dword sequence. If the vCPU halts during
> > > > +	 * save/restore while this sequence is being issued, partial writes may trigger
> > > > +	 * page faults when saving iGPU CCS metadata. Use the VMOVDQU instruction to
> > > > +	 * write the sequence atomically.
> > > > +	 */
> > > > +	emit_atomic(gt, cs, dw, sizeof(dw));
> > > > +	cs += EMIT_COPY_CCS_DW;
> > > >  	bb->len = cs - bb->cs;
> > > >  }
> > > >  
> > > > @@ -993,18 +1035,27 @@ static u64 migrate_vm_ppgtt_addr_tlb_inval(void)
> > > >  	return (NUM_KERNEL_PDE - 2) * XE_PAGE_SIZE;
> > > >  }
> > > >  
> > > > -static int emit_flush_invalidate(u32 *dw, int i, u32 flags)
> > > > +/*
> > > > + * The MI_FLUSH_DW command is a 4-dword sequence. If the vCPU halts during
> > > > + * save/restore while this sequence is being issued, partial writes may
> > > > + * trigger page faults when saving iGPU CCS metadata. Use
> > > > + * emit_atomic() to write the sequence atomically.
> > > > + */
> > > > +#define EMIT_FLUSH_INVALIDATE_DW 4
> > > > +static int emit_flush_invalidate(struct xe_exec_queue *q, u32 *cs, int i, u32 flags)
> > > >  {
> > > >  	u64 addr = migrate_vm_ppgtt_addr_tlb_inval();
> > > > +	u32 dw[EMIT_FLUSH_INVALIDATE_DW] = {MI_NOOP}, j = 0;
> > > > +
> > > > +	dw[j++] = MI_FLUSH_DW | MI_INVALIDATE_TLB | MI_FLUSH_DW_OP_STOREDW |
> > > > +		      MI_FLUSH_IMM_DW | flags;
> > > > +	dw[j++] = lower_32_bits(addr);
> > > > +	dw[j++] = upper_32_bits(addr);
> > > > +	dw[j++] = MI_NOOP;
> > > >  
> > > > -	dw[i++] = MI_FLUSH_DW | MI_INVALIDATE_TLB | MI_FLUSH_DW_OP_STOREDW |
> > > > -		  MI_FLUSH_IMM_DW | flags;
> > > > -	dw[i++] = lower_32_bits(addr);
> > > > -	dw[i++] = upper_32_bits(addr);
> > > > -	dw[i++] = MI_NOOP;
> > > > -	dw[i++] = MI_NOOP;
> > > > +	emit_atomic(q->gt, &cs[i], dw, sizeof(dw));
> > > >  
> > > > -	return i;
> > > > +	return i + j;
> > > >  }
> > > >  
> > > >  /**
> > > > @@ -1049,7 +1100,7 @@ int xe_migrate_ccs_rw_copy(struct xe_tile *tile, struct xe_exec_queue *q,
> > > >  	/* Calculate Batch buffer size */
> > > >  	batch_size = 0;
> > > >  	while (size) {
> > > > -		batch_size += 10; /* Flush + ggtt addr + 2 NOP */
> > > > +		batch_size += EMIT_FLUSH_INVALIDATE_DW * 2; /* Flush + ggtt addr + 1 NOP */
> > > >  		u64 ccs_ofs, ccs_size;
> > > >  		u32 ccs_pt;
> > > >  
> > > > @@ -1090,7 +1141,7 @@ int xe_migrate_ccs_rw_copy(struct xe_tile *tile, struct xe_exec_queue *q,
> > > >  	 * sizes here again before copy command is emitted.
> > > >  	 */
> > > >  	while (size) {
> > > > -		batch_size += 10; /* Flush + ggtt addr + 2 NOP */
> > > > +		batch_size += EMIT_FLUSH_INVALIDATE_DW * 2; /* Flush + ggtt addr + 1 NOP */
> > > >  		u32 flush_flags = 0;
> > > >  		u64 ccs_ofs, ccs_size;
> > > >  		u32 ccs_pt;
> > > > @@ -1113,11 +1164,11 @@ int xe_migrate_ccs_rw_copy(struct xe_tile *tile, struct xe_exec_queue *q,
> > > >  
> > > >  		emit_pte(m, bb, ccs_pt, false, false, &ccs_it, ccs_size, src);
> > > >  
> > > > -		bb->len = emit_flush_invalidate(bb->cs, bb->len, flush_flags);
> > > > +		bb->len = emit_flush_invalidate(q, bb->cs, bb->len, flush_flags);
> > > >  		flush_flags = xe_migrate_ccs_copy(m, bb, src_L0_ofs, src_is_pltt,
> > > >  						  src_L0_ofs, dst_is_pltt,
> > > >  						  src_L0, ccs_ofs, true);
> > > > -		bb->len = emit_flush_invalidate(bb->cs, bb->len, flush_flags);
> > > > +		bb->len = emit_flush_invalidate(q, bb->cs, bb->len, flush_flags);
> > > >  
> > > >  		size -= src_L0;
> > > >  	}
> > > > -- 
> > > > 2.51.0
> > > >