Re: [PATCH v4 6/9] lib: sbi_hart_worlds: Implement WID hart protection backend

[Yu-Chien Peter Lin <peter.lin@sifive.com>]

On Fri, Oct 30, 2026 at 06:45:15PM +0800, Yu-Chien Peter Lin wrote:
> Introduce a WID-based hart protection backend that switches World ID
> configurations across domains.
> 
> Access to slwid is gated by mwiddeleg (Sswid), which means slwid cannot
> be handled by generic hart_context save/restore. Instead, manage slwid as
> domain state with explicit sequencing:
> 
> - configure:
>   - Validate and program mlwid and mwiddeleg according to domain policy
>     and platform constraints (pmlwidlist/mlwidlist).
>   - Restore slwid after mwiddeleg is enabled.
> - unconfigure:
>   - Save slwid prior to clearing mwiddeleg.
>   - Reset mwiddeleg to 0 to disable Sswid.
> 
> Signed-off-by: Yu-Chien Peter Lin <peter.lin@sifive.com>
> Reviewed-by: Pawandeep Oza <pawandeep.oza@oss.qualcomm.com>
> Reviewed-by: Anup Patel <anup@brainfault.org>
> ---
>  include/sbi/sbi_hart_worlds.h |  24 ++++
>  lib/sbi/objects.mk            |   1 +
>  lib/sbi/sbi_hart.c            |   5 +
>  lib/sbi/sbi_hart_worlds.c     | 210 ++++++++++++++++++++++++++++++++++
>  4 files changed, 240 insertions(+)
>  create mode 100644 include/sbi/sbi_hart_worlds.h
>  create mode 100644 lib/sbi/sbi_hart_worlds.c
> 
> diff --git a/include/sbi/sbi_hart_worlds.h b/include/sbi/sbi_hart_worlds.h
> new file mode 100644
> index 00000000..91b11023
> --- /dev/null
> +++ b/include/sbi/sbi_hart_worlds.h
> @@ -0,0 +1,24 @@
> +/*
> + * SPDX-License-Identifier: BSD-2-Clause
> + *
> + * Copyright (c) 2026 SiFive Inc.
> + */
> +
> +#ifndef __SBI_HART_WORLDS_H__
> +#define __SBI_HART_WORLDS_H__
> +
> +struct sbi_scratch;
> +
> +/**
> + * Initialize WID hart protection for current HART
> + *
> + * Registers the WID protection mechanism if Smlwid
> + * extension is present.
> + *
> + * @param scratch pointer to scratch space of current HART
> + *
> + * @return 0 on success and negative error code on failure
> + */
> +int sbi_hart_worlds_init(struct sbi_scratch *scratch);
> +
> +#endif /* __SBI_HART_WORLDS_H__ */
> diff --git a/lib/sbi/objects.mk b/lib/sbi/objects.mk
> index ae27d0c5..355ae6e9 100644
> --- a/lib/sbi/objects.mk
> +++ b/lib/sbi/objects.mk
> @@ -77,6 +77,7 @@ libsbi-objs-y += sbi_fwft.o
>  libsbi-objs-y += sbi_hart.o
>  libsbi-objs-y += sbi_hart_pmp.o
>  libsbi-objs-y += sbi_hart_protection.o
> +libsbi-objs-y += sbi_hart_worlds.o
>  libsbi-objs-y += sbi_heap.o
>  libsbi-objs-y += sbi_math.o
>  libsbi-objs-y += sbi_hfence.o
> diff --git a/lib/sbi/sbi_hart.c b/lib/sbi/sbi_hart.c
> index 611c97bf..f7408f51 100644
> --- a/lib/sbi/sbi_hart.c
> +++ b/lib/sbi/sbi_hart.c
> @@ -17,6 +17,7 @@
>  #include <sbi/sbi_error.h>
>  #include <sbi/sbi_hart.h>
>  #include <sbi/sbi_hart_pmp.h>
> +#include <sbi/sbi_hart_worlds.h>
>  #include <sbi/sbi_platform.h>
>  #include <sbi/sbi_pmu.h>
>  #include <sbi/sbi_string.h>
> @@ -742,6 +743,10 @@ int sbi_hart_init(struct sbi_scratch *scratch, bool cold_boot)
>  		rc = sbi_hart_pmp_init(scratch);
>  		if (rc)
>  			return rc;
> +
> +		rc = sbi_hart_worlds_init(scratch);
> +		if (rc)
> +			return rc;
>  	}
>  
>  	return sbi_hart_reinit(scratch);
> diff --git a/lib/sbi/sbi_hart_worlds.c b/lib/sbi/sbi_hart_worlds.c
> new file mode 100644
> index 00000000..6f582bd2
> --- /dev/null
> +++ b/lib/sbi/sbi_hart_worlds.c
> @@ -0,0 +1,210 @@
> +/*
> + * SPDX-License-Identifier: BSD-2-Clause
> + *
> + * Copyright (c) 2026 SiFive Inc.
> + */
> +
> +#include <sbi/riscv_encoding.h>
> +#include <sbi/sbi_bitops.h>
> +#include <sbi/sbi_console.h>
> +#include <sbi/sbi_domain.h>
> +#include <sbi/sbi_error.h>
> +#include <sbi/sbi_hart.h>
> +#include <sbi/sbi_hart_protection.h>
> +#include <sbi/sbi_hart_worlds.h>
> +#include <sbi/sbi_scratch.h>
> +
> +/**
> + * Initialize the per-hart slwid values of a domain using
> + * the lowest delegated WID as the initial value.
> + *
> + * S-mode OS should choose the desire slwid for it's
> + * U-mode.
> + */
> +static int sbi_hart_worlds_state_setup(struct sbi_domain *dom,
> +				       struct sbi_domain_state *state,
> +				       void *state_ptr)
> +{
> +	unsigned long widdeleg = dom->widdeleg;
> +	unsigned long *slwid = state_ptr;
> +	unsigned long init_wid;
> +
> +	if (!widdeleg)
> +		return SBI_OK;
> +
> +	init_wid = sbi_ffs(widdeleg);
> +	sbi_for_each_hartindex(i)
> +		slwid[i] = init_wid;
> +
> +	return SBI_OK;
> +}
> +
> +static struct sbi_domain_state wid_state = {
> +	.state_setup = sbi_hart_worlds_state_setup,
> +};
> +
> +static unsigned long *sbi_hart_worlds_slwid_ptr(struct sbi_domain *dom)
> +{
> +	unsigned long *slwid;
> +
> +	slwid = sbi_domain_state_ptr(dom, &wid_state);
> +	if (!slwid || !sbi_hartindex_valid(current_hartindex()))
> +		return NULL;
> +
> +	return &slwid[current_hartindex()];
> +}
> +
> +static int hart_worlds_validate_wid(struct sbi_scratch *scratch,
> +				    struct sbi_domain *dom,
> +				    unsigned long mlwidlist)
> +{
> +	struct sbi_hart_features *hf = sbi_hart_features_ptr(scratch);
> +
> +	if (hf->has_pmlwidlist && !(hf->pmlwidlist & BIT_ULL(dom->wid))) {
> +		sbi_printf("%s: domain%d WID %u not permitted by"
> +			   " pmlwidlist\n",
> +			   __func__, dom->index, dom->wid);
> +		return SBI_EINVAL;
> +	}
> +
> +	if (sbi_hart_has_extension(scratch, SBI_HART_EXT_SMLWIDLIST) &&
> +	    !(mlwidlist & BIT_ULL(dom->wid))) {
> +		sbi_printf("%s: domain%d WID %u not permitted by"
> +			   " mlwidlist 0x%lx\n",
> +			   __func__, dom->index, dom->wid, mlwidlist);
> +		return SBI_EINVAL;
> +	}
> +
> +	return SBI_OK;
> +}
> +
> +/*
> + * Validate that a domain's widdeleg value, parsed from the
> + * "widdeleg" DT property, is permitted on this hart.
> + *
> + * mwiddeleg is a M-mode WARL register, so an S-mode OS must rely
> + * on the "widdeleg" DT property to determine which WIDs it may
> + * delegate to U-mode via slwid. If pmwidlist or mlwidlist silently
> + * filters out any of those WIDs, the S-mode OS's view of the WIDs
> + * available for delegation would be incorrect. Treat this mismatch
> + * as a misconfiguration.
> + */
> +static int hart_worlds_validate_widdeleg(struct sbi_scratch *scratch,
> +					 struct sbi_domain *dom,
> +					 unsigned long mlwidlist)
> +{
> +	struct sbi_hart_features *hf = sbi_hart_features_ptr(scratch);
> +	unsigned long widdeleg = dom->widdeleg;
> +
> +	if (!widdeleg)
> +		return SBI_OK;
> +
> +	if (hf->has_pmlwidlist && (widdeleg & ~hf->pmlwidlist)) {
> +		sbi_printf("%s: domain%d widdeleg 0x%lx has WID(s)"
> +			   " outside pmlwidlist 0x%lx\n",
> +			   __func__, dom->index, widdeleg,
> +			   (unsigned long)hf->pmlwidlist);
> +		return SBI_EINVAL;
> +	}
> +
> +	if (sbi_hart_has_extension(scratch, SBI_HART_EXT_SMLWIDLIST) &&
> +	    (widdeleg & ~mlwidlist)) {
> +		sbi_printf("%s: domain%d widdeleg 0x%lx has WID(s)"
> +			   " outside mlwidlist 0x%lx\n",
> +			   __func__, dom->index, widdeleg, mlwidlist);
> +		return SBI_EINVAL;
> +	}
> +
> +	return SBI_OK;
> +}
> +
> +static int sbi_hart_worlds_configure(struct sbi_scratch *scratch,
> +				     struct sbi_domain *dom)
> +{
> +	unsigned long *slwid = NULL;
> +	unsigned long mlwidlist = 0;
> +	int rc;
> +
> +	if (!sbi_hart_has_extension(scratch, SBI_HART_EXT_SMLWID))
> +		return SBI_OK;
> +
> +	if (sbi_hart_has_extension(scratch, SBI_HART_EXT_SMLWIDLIST))
> +		mlwidlist = csr_read(CSR_MLWIDLIST);
> +
> +	rc = hart_worlds_validate_wid(scratch, dom, mlwidlist);
> +	if (rc)
> +		return rc;
> +
> +	csr_write(CSR_MLWID, dom->wid);

Need to validate widdelg and bail out before updating mlwid, so mlwid
isn't left holding an invalid target world state on failure. Will fix
in PATCHv5.

Regards,
Peter Lin

> +	rc = hart_worlds_validate_widdeleg(scratch, dom, mlwidlist);
> +	if (rc)
> +		return rc;
> +
> +	/*
> +	 * Enable Sswid before restoring slwid. A zero mwiddeleg
> +	 * disables access to slwid, so the order here is required.
> +	 */
> +	csr_write(CSR_MWIDDELEG, dom->widdeleg);
> +
> +	if (dom->widdeleg) {
> +		slwid = sbi_hart_worlds_slwid_ptr(dom);
> +		if (!slwid)
> +			return SBI_EINVAL;
> +		csr_write(CSR_SLWID, *slwid);
> +	}
> +
> +	return SBI_OK;
> +}

-- 
opensbi mailing list
opensbi@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/opensbi