Subject: Re: [PATCH v14 10/44] arm64: RMI: Add support for SRO

From: Marc Zyngier <[email-masked]>
Date: Thu, 21 May 2026 15:35:41 +0100

On Wed, 13 May 2026 14:17:18 +0100,
Steven Price <[email-masked]> wrote:
> 
> RMM v2.0 introduces the concept of "Stateful RMI Operations" (SRO). This
> means that an SMC can return with an operation still in progress. The
> host is excepted to continue the operation until is reaches a conclusion
> (either success or failure). During this process the RMM can request
> additional memory ('donate') or hand memory back to the host
> ('reclaim'). The host can request an in progress operation is cancelled,
> but still continue the operation until it has completed (otherwise the
> incomplete operation may cause future RMM operations to fail).
> 
> The SRO is tracked using a struct rmi_sro_state object which keeps track
> of any memory which has been allocated but not yet consumed by the RMM
> or reclaimed from the RMM. This allows the memory to be reused in a
> future request within the same operation. It will also permit an
> operation to be done in a context where memory allocation may be
> difficult (e.g. atomic context) with the option to abort the operation
> and retry the memory allocation outside of the atomic context. The
> memory stored in the struct rmi_sro_state object can then be reused on
> the subsequent attempt.
> 
> Signed-off-by: Steven Price <[email-masked]>
> ---
> v14:
>  * SRO support has improved although is still not fully complete. The
>    infrastructure has been moved out of KVM.
> ---
>  arch/arm64/include/asm/rmi_cmds.h |   1 +
>  arch/arm64/kernel/rmi.c           | 359 ++++++++++++++++++++++++++++++
>  2 files changed, 360 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/rmi_cmds.h b/arch/arm64/include/asm/rmi_cmds.h
> index eb213c8e6f26..1a7b0c8f1e38 100644
> --- a/arch/arm64/include/asm/rmi_cmds.h
> +++ b/arch/arm64/include/asm/rmi_cmds.h
> @@ -35,6 +35,7 @@ struct rmi_sro_state {
>  
>  int rmi_delegate_range(phys_addr_t phys, unsigned long size);
>  int rmi_undelegate_range(phys_addr_t phys, unsigned long size);
> +int free_delegated_page(phys_addr_t phys);
>  
>  static inline int rmi_delegate_page(phys_addr_t phys)
>  {
> diff --git a/arch/arm64/kernel/rmi.c b/arch/arm64/kernel/rmi.c
> index 08cef54acadb..a8107ca9bb6d 100644
> --- a/arch/arm64/kernel/rmi.c
> +++ b/arch/arm64/kernel/rmi.c
> @@ -48,6 +48,365 @@ int rmi_undelegate_range(phys_addr_t phys, unsigned long size)
>  	return ret;
>  }
>  
> +static unsigned long donate_req_to_size(unsigned long donatereq)
> +{
> +	unsigned long unit_size = RMI_DONATE_SIZE(donatereq);
> +
> +	switch (unit_size) {
> +	case 0:
> +		return PAGE_SIZE;
> +	case 1:
> +		return PMD_SIZE;
> +	case 2:
> +		return PUD_SIZE;
> +	case 3:
> +		return P4D_SIZE;

How does this work when we have folded levels? If this is supposed to
be the architected size, then it should actively express that:

	return BIT(unit_size * (PAGE_SHIFT - 3) + PAGE_SHIFT);

> +	}
> +	unreachable();
> +}
> +
> +static void rmi_smccc_invoke(struct arm_smccc_1_2_regs *regs_in,
> +			     struct arm_smccc_1_2_regs *regs_out)
> +{
> +	struct arm_smccc_1_2_regs regs = *regs_in;
> +	unsigned long status;
> +
> +	do {
> +		arm_smccc_1_2_invoke(&regs, regs_out);
> +		status = RMI_RETURN_STATUS(regs_out->a0);
> +	} while (status == RMI_BUSY || status == RMI_BLOCKED);
> +}
> +
> +int free_delegated_page(phys_addr_t phys)
> +{
> +	if (WARN_ON(rmi_undelegate_page(phys))) {

Please drop this WARN_ON(). Or at least make it ONCE. Everywhere.

> +		/* Undelegate failed: leak the page */
> +		return -EBUSY;
> +	}
> +
> +	free_page((unsigned long)phys_to_virt(phys));
> +
> +	return 0;
> +}
> +
> +static int rmi_sro_ensure_capacity(struct rmi_sro_state *sro,
> +				   unsigned long count)
> +{
> +	if (WARN_ON_ONCE(sro->addr_count > RMI_MAX_ADDR_LIST))
> +		return -EOVERFLOW;
> +
> +	if (count > RMI_MAX_ADDR_LIST - sro->addr_count)
> +		return -ENOSPC;
> +
> +	return 0;
> +}
> +
> +static int rmi_sro_donate_contig(struct rmi_sro_state *sro,
> +				 unsigned long sro_handle,
> +				 unsigned long donatereq,
> +				 struct arm_smccc_1_2_regs *out_regs,
> +				 gfp_t gfp)
> +{
> +	unsigned long unit_size = RMI_DONATE_SIZE(donatereq);
> +	unsigned long unit_size_bytes = donate_req_to_size(donatereq);
> +	unsigned long count = RMI_DONATE_COUNT(donatereq);
> +	unsigned long state = RMI_DONATE_STATE(donatereq);
> +	unsigned long size = unit_size_bytes * count;
> +	unsigned long addr_range;
> +	int ret;
> +	void *virt;
> +	phys_addr_t phys;
> +	struct arm_smccc_1_2_regs regs = {
> +		SMC_RMI_OP_MEM_DONATE,
> +		sro_handle
> +	};
> +
> +	for (int i = 0; i < sro->addr_count; i++) {
> +		unsigned long entry = sro->addr_list[i];
> +
> +		if (RMI_ADDR_RANGE_SIZE(entry) == unit_size &&
> +		    RMI_ADDR_RANGE_COUNT(entry) == count &&
> +		    RMI_ADDR_RANGE_STATE(entry) == state) {
> +			sro->addr_count--;
> +			swap(sro->addr_list[sro->addr_count],
> +			     sro->addr_list[i]);
> +
> +			goto out;
> +		}
> +	}
> +
> +	ret = rmi_sro_ensure_capacity(sro, 1);
> +	if (ret)
> +		return ret;
> +
> +	virt = alloc_pages_exact(size, gfp);
> +	if (!virt)
> +		return -ENOMEM;
> +	phys = virt_to_phys(virt);
> +
> +	if (state == RMI_OP_MEM_DELEGATED) {
> +		if (rmi_delegate_range(phys, size)) {
> +			free_pages_exact(virt, size);
> +			return -ENXIO;
> +		}
> +	}
> +
> +	addr_range = phys & RMI_ADDR_RANGE_ADDR_MASK;
> +	FIELD_MODIFY(RMI_ADDR_RANGE_SIZE_MASK, &addr_range, unit_size);
> +	FIELD_MODIFY(RMI_ADDR_RANGE_COUNT_MASK, &addr_range, count);
> +	FIELD_MODIFY(RMI_ADDR_RANGE_STATE_MASK, &addr_range, state);
> +
> +	sro->addr_list[sro->addr_count] = addr_range;
> +

Shouldn't this be moved to a helper that ensures capacity, and returns
an error otherwise?

> +out:
> +	regs.a2 = virt_to_phys(&sro->addr_list[sro->addr_count]);
> +	regs.a3 = 1;

This could really do with context specific helpers that populate regs
based on a set of parameters. I have no idea what this 1 here is, and
the init is spread over too much code. Think of the children!

That's valid for the whole patch.

	M.
> +	rmi_smccc_invoke(&regs, out_regs);
> +
> +	unsigned long donated_granules = out_regs->a1;
> +	unsigned long donated_size = donated_granules << PAGE_SHIFT;
> +
> +	if (donated_granules == 0) {
> +		/* No pages used by the RMM */
> +		sro->addr_count++;
> +	} else if (donated_size < size) {
> +		phys = sro->addr_list[sro->addr_count] & RMI_ADDR_RANGE_ADDR_MASK;
> +
> +		/* Not all granules used by the RMM, free the remaining pages */
> +		for (long i = donated_size; i < size; i += PAGE_SIZE) {
> +			if (state == RMI_OP_MEM_DELEGATED)
> +				free_delegated_page(phys + i);
> +			else
> +				__free_page(phys_to_page(phys + i));
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +static int rmi_sro_donate_noncontig(struct rmi_sro_state *sro,
> +				    unsigned long sro_handle,
> +				    unsigned long donatereq,
> +				    struct arm_smccc_1_2_regs *out_regs,
> +				    gfp_t gfp)
> +{
> +	unsigned long unit_size = RMI_DONATE_SIZE(donatereq);
> +	unsigned long unit_size_bytes = donate_req_to_size(donatereq);
> +	unsigned long count = RMI_DONATE_COUNT(donatereq);
> +	unsigned long state = RMI_DONATE_STATE(donatereq);
> +	unsigned long found = 0;
> +	unsigned long addr_list_start = sro->addr_count;
> +	int ret;
> +	struct arm_smccc_1_2_regs regs = {
> +		SMC_RMI_OP_MEM_DONATE,
> +		sro_handle
> +	};
> +
> +	for (int i = 0; i < addr_list_start && found < count; i++) {
> +		unsigned long entry = sro->addr_list[i];
> +
> +		if (RMI_ADDR_RANGE_SIZE(entry) == unit_size &&
> +		    RMI_ADDR_RANGE_COUNT(entry) == 1 &&
> +		    RMI_ADDR_RANGE_STATE(entry) == state) {
> +			addr_list_start--;
> +			swap(sro->addr_list[addr_list_start],
> +			     sro->addr_list[i]);
> +			found++;
> +			i--;
> +		}
> +	}
> +
> +	ret = rmi_sro_ensure_capacity(sro, count - found);
> +	if (ret)
> +		return ret;
> +
> +	while (found < count) {
> +		unsigned long addr_range;
> +		void *virt = alloc_pages_exact(unit_size_bytes, gfp);
> +		phys_addr_t phys;
> +
> +		if (!virt)
> +			return -ENOMEM;
> +
> +		phys = virt_to_phys(virt);
> +
> +		if (state == RMI_OP_MEM_DELEGATED) {
> +			if (rmi_delegate_range(phys, unit_size_bytes)) {
> +				free_pages_exact(virt, unit_size_bytes);
> +				return -ENXIO;
> +			}
> +		}
> +
> +		addr_range = phys & RMI_ADDR_RANGE_ADDR_MASK;
> +		FIELD_MODIFY(RMI_ADDR_RANGE_SIZE_MASK, &addr_range, unit_size);
> +		FIELD_MODIFY(RMI_ADDR_RANGE_COUNT_MASK, &addr_range, 1);
> +		FIELD_MODIFY(RMI_ADDR_RANGE_STATE_MASK, &addr_range, state);
> +
> +		sro->addr_list[sro->addr_count++] = addr_range;
> +		found++;
> +	}
> +
> +	regs.a2 = virt_to_phys(&sro->addr_list[addr_list_start]);
> +	regs.a3 = found;
> +	rmi_smccc_invoke(&regs, out_regs);
> +
> +	unsigned long donated_granules = out_regs->a1;
> +
> +	if (WARN_ON(donated_granules & ((unit_size_bytes >> PAGE_SHIFT) - 1))) {
> +		/*
> +		 * FIXME: RMM has only consumed part of a huge page, this leaks
> +		 * the rest of the huge page
> +		 */
> +		donated_granules = ALIGN(donated_granules,
> +					 (unit_size_bytes >> PAGE_SHIFT));
> +	}
> +	unsigned long donated_blocks = donated_granules / (unit_size_bytes >> PAGE_SHIFT);
> +
> +	if (WARN_ON(donated_blocks > found))
> +		donated_blocks = found;
> +
> +	unsigned long undonated_blocks = found - donated_blocks;
> +
> +	while (donated_blocks && undonated_blocks) {
> +		sro->addr_count--;
> +		swap(sro->addr_list[addr_list_start],
> +		     sro->addr_list[sro->addr_count]);
> +		addr_list_start++;
> +
> +		donated_blocks--;
> +		undonated_blocks--;
> +	}
> +	sro->addr_count -= donated_blocks;
> +
> +	return 0;
> +}
> +
> +static int rmi_sro_donate(struct rmi_sro_state *sro,
> +			  unsigned long sro_handle,
> +			  unsigned long donatereq,
> +			  struct arm_smccc_1_2_regs *regs,
> +			  gfp_t gfp)
> +{
> +	unsigned long count = RMI_DONATE_COUNT(donatereq);
> +
> +	if (WARN_ON(!count))
> +		return 0;
> +
> +	if (RMI_DONATE_CONTIG(donatereq)) {
> +		return rmi_sro_donate_contig(sro, sro_handle, donatereq,
> +					     regs, gfp);
> +	} else {
> +		return rmi_sro_donate_noncontig(sro, sro_handle, donatereq,
> +						regs, gfp);
> +	}
> +}
> +
> +static int rmi_sro_reclaim(struct rmi_sro_state *sro,
> +			   unsigned long sro_handle,
> +			   struct arm_smccc_1_2_regs *out_regs)
> +{
> +	unsigned long capacity;
> +	struct arm_smccc_1_2_regs regs;
> +	int ret;
> +
> +	ret = rmi_sro_ensure_capacity(sro, 1);
> +	if (ret)
> +		rmi_sro_free(sro);
> +
> +	capacity = RMI_MAX_ADDR_LIST - sro->addr_count;
> +
> +	regs = (struct arm_smccc_1_2_regs){
> +		SMC_RMI_OP_MEM_RECLAIM,
> +		sro_handle,
> +		virt_to_phys(&sro->addr_list[sro->addr_count]),
> +		capacity
> +	};
> +	rmi_smccc_invoke(&regs, out_regs);
> +
> +	if (WARN_ON_ONCE(out_regs->a1 > capacity))
> +		out_regs->a1 = capacity;
> +
> +	sro->addr_count += out_regs->a1;
> +
> +	return 0;
> +}
> +
> +void rmi_sro_free(struct rmi_sro_state *sro)
> +{
> +	for (int i = 0; i < sro->addr_count; i++) {
> +		unsigned long entry = sro->addr_list[i];
> +		unsigned long addr = RMI_ADDR_RANGE_ADDR(entry);
> +		unsigned long unit_size = RMI_ADDR_RANGE_SIZE(entry);
> +		unsigned long count = RMI_ADDR_RANGE_COUNT(entry);
> +		unsigned long state = RMI_ADDR_RANGE_STATE(entry);
> +		unsigned long size = donate_req_to_size(unit_size) * count;
> +
> +		if (state == RMI_OP_MEM_DELEGATED) {
> +			if (WARN_ON(rmi_undelegate_range(addr, size))) {
> +				/* Leak the pages */
> +				continue;
> +			}
> +		}
> +		free_pages_exact(phys_to_virt(addr), size);
> +	}
> +
> +	sro->addr_count = 0;
> +}
> +
> +unsigned long rmi_sro_execute(struct rmi_sro_state *sro, gfp_t gfp)
> +{
> +	unsigned long sro_handle;
> +	struct arm_smccc_1_2_regs regs;
> +	struct arm_smccc_1_2_regs *regs_in = &sro->regs;
> +
> +	rmi_smccc_invoke(regs_in, &regs);
> +
> +	sro_handle = regs.a1;
> +
> +	while (RMI_RETURN_STATUS(regs.a0) == RMI_INCOMPLETE) {
> +		bool can_cancel = RMI_RETURN_CAN_CANCEL(regs.a0);
> +		int ret;
> +
> +		switch (RMI_RETURN_MEMREQ(regs.a0)) {
> +		case RMI_OP_MEM_REQ_NONE:
> +			regs = (struct arm_smccc_1_2_regs){
> +				SMC_RMI_OP_CONTINUE, sro_handle, 0
> +			};
> +			rmi_smccc_invoke(&regs, &regs);
> +			break;
> +		case RMI_OP_MEM_REQ_DONATE:
> +			ret = rmi_sro_donate(sro, sro_handle, regs.a2, &regs,
> +					     gfp);
> +			break;
> +		case RMI_OP_MEM_REQ_RECLAIM:
> +			ret = rmi_sro_reclaim(sro, sro_handle, &regs);
> +			break;
> +		default:
> +			ret = WARN_ON(1);
> +			break;
> +		}
> +
> +		if (ret) {
> +			if (can_cancel) {
> +				/*
> +				 * FIXME: Handle cancelling properly!
> +				 *
> +				 * If the operation has failed due to memory
> +				 * allocation failure then the information on
> +				 * the memory allocation should be saved, so
> +				 * that the allocation can be repeated outside
> +				 * of any context which prevented the
> +				 * allocation.

Honestly, this is the sort of stuff that I'd expect to be solved
*before* posting this code. Since this is so central to the whole
memory management, it needs to be correct from day-1.

If you can't make it work in time, then tone the supported features
down. But FIXMEs and WARN_ONs are not the way to go.

	M.

-- 
Without deviation from the norm, progress is not possible.