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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [RFC 09/16] xen/arm: p2m: Introduce a function to resolve translation fault
On Mon, 8 Oct 2018, Julien Grall wrote:
> Currently a Stage-2 translation fault could happen:
> 1) MMIO emulation
> 2) When the page-tables is been updated using Break-Before-Make
^ have
> 3) Page not mapped
>
> A follow-up patch will re-purpose the valid bit in an entry to generate
> translation fault. This would be used to do an action on each entries to
^entry
> track page used for a given period.
^pages
>
> A new function is introduced to try to resolve a translation fault. This
> will include 2) and the new way to generate fault explained above.
I can see the code does what you describe, but I don't understand why we
are doing this. What is missing in the commit message is the explanation
of the relationship between the future goal of repurposing the valid bit
and the introduction of a function to handle Break-Before-Make stage2
faults. Does it fix an issue with Break-Before-Make that we currently
have? Or it becomes needed due to the repurposing of valid? If so, why?
> To avoid invalidating all the page-tables entries in one go. It is
> possible to invalidate the top-level table and then on trap invalidate
> the table one-level down. This will be repeated until a block/page entry
> has been reached.
>
> At the moment, there are no action done when reaching a block/page entry
> but setting the valid bit to 1.
>
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> ---
> xen/arch/arm/p2m.c | 127
> +++++++++++++++++++++++++++++++++++++++++++++++++++
> xen/arch/arm/traps.c | 7 +--
> 2 files changed, 131 insertions(+), 3 deletions(-)
>
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index ec956bc151..af445d3313 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1043,6 +1043,133 @@ int p2m_set_entry(struct p2m_domain *p2m,
> return rc;
> }
>
> +/* Invalidate all entries in the table. The p2m should be write locked. */
> +static void p2m_invalidate_table(struct p2m_domain *p2m, mfn_t mfn)
> +{
> + lpae_t *table;
> + unsigned int i;
> +
> + ASSERT(p2m_is_write_locked(p2m));
> +
> + table = map_domain_page(mfn);
> +
> + for ( i = 0; i < LPAE_ENTRIES; i++ )
> + {
> + lpae_t pte = table[i];
> +
> + pte.p2m.valid = 0;
> +
> + p2m_write_pte(&table[i], pte, p2m->clean_pte);
> + }
> +
> + unmap_domain_page(table);
> +
> + p2m->need_flush = true;
> +}
> +
> +bool p2m_resolve_translation_fault(struct domain *d, gfn_t gfn)
> +{
> + struct p2m_domain *p2m = p2m_get_hostp2m(d);
> + unsigned int level = 0;
> + bool resolved = false;
> + lpae_t entry, *table;
> + paddr_t addr = gfn_to_gaddr(gfn);
> +
> + /* Convenience aliases */
> + const unsigned int offsets[4] = {
> + zeroeth_table_offset(addr),
> + first_table_offset(addr),
> + second_table_offset(addr),
> + third_table_offset(addr)
> + };
> +
> + p2m_write_lock(p2m);
> +
> + /* This gfn is higher than the highest the p2m map currently holds */
> + if ( gfn_x(gfn) > gfn_x(p2m->max_mapped_gfn) )
> + goto out;
> +
> + table = p2m_get_root_pointer(p2m, gfn);
> + /*
> + * The table should always be non-NULL because the gfn is below
> + * p2m->max_mapped_gfn and the root table pages are always present.
> + */
> + BUG_ON(table == NULL);
> +
> + /*
> + * Go down the page-tables until an entry has the valid bit unset or
> + * a block/page entry has been hit.
> + */
> + for ( level = P2M_ROOT_LEVEL; level <= 3; level++ )
> + {
> + int rc;
> +
> + entry = table[offsets[level]];
> +
> + if ( level == 3 )
> + break;
> +
> + /* Stop as soon as we hit an entry with the valid bit unset. */
> + if ( !lpae_is_valid(entry) )
> + break;
> +
> + rc = p2m_next_level(p2m, true, level, &table, offsets[level]);
> + if ( rc == GUEST_TABLE_MAP_FAILED )
> + goto out_unmap;
> + else if ( rc != GUEST_TABLE_NORMAL_PAGE )
why not rc == GUEST_TABLE_SUPER_PAGE?
> + break;
> + }
> +
> + /*
> + * If the valid bit of the entry is set, it means someone was playing
> with
> + * the Stage-2 page table. Nothing to do and mark the fault as resolved.
> + */
> + if ( lpae_is_valid(entry) )
> + {
> + resolved = true;
> + goto out_unmap;
> + }
> +
> + /*
> + * The valid bit is unset. If the entry is still not valid then the fault
> + * cannot be resolved, exit and report it.
> + */
> + if ( !p2m_is_valid(entry) )
> + goto out_unmap;
> +
> + /*
> + * Now we have an entry with valid bit unset, but still valid from
> + * the P2M point of view.
> + *
> + * For entry pointing to a table, the table will be invalidated.
^ entries
> + * For entry pointing to a block/page, no work to do for now.
^ entries
> + */
> + if ( lpae_is_table(entry, level) )
> + p2m_invalidate_table(p2m, lpae_get_mfn(entry));
Maybe because I haven't read the rest of the patches, it is not clear to
me why in the case of an entry pointing to a table we need to invalidate
it, and otherwise set valid to 1.
> + /*
> + * Now that the work on the entry is done, set the valid bit to prevent
> + * another fault on that entry.
> + */
> + resolved = true;
> + entry.p2m.valid = 1;
> +
> + p2m_write_pte(table + offsets[level], entry, p2m->clean_pte);
> +
> + /*
> + * No need to flush the TLBs as the modified entry had the valid bit
> + * unset.
> + */
> +
> +out_unmap:
> + unmap_domain_page(table);
> +
> +out:
> + p2m_write_unlock(p2m);
> +
> + return resolved;
> +}
> +
> static inline int p2m_insert_mapping(struct domain *d,
> gfn_t start_gfn,
> unsigned long nr,
> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> index b40798084d..169b57cb6b 100644
> --- a/xen/arch/arm/traps.c
> +++ b/xen/arch/arm/traps.c
> @@ -1882,6 +1882,8 @@ static bool try_map_mmio(gfn_t gfn)
> return !map_regions_p2mt(d, gfn, 1, mfn, p2m_mmio_direct_c);
> }
>
> +bool p2m_resolve_translation_fault(struct domain *d, gfn_t gfn);
Should be in an header file?
> static void do_trap_stage2_abort_guest(struct cpu_user_regs *regs,
> const union hsr hsr)
> {
> @@ -1894,7 +1896,6 @@ static void do_trap_stage2_abort_guest(struct
> cpu_user_regs *regs,
> vaddr_t gva;
> paddr_t gpa;
> uint8_t fsc = xabt.fsc & ~FSC_LL_MASK;
> - mfn_t mfn;
> bool is_data = (hsr.ec == HSR_EC_DATA_ABORT_LOWER_EL);
>
> /*
> @@ -1977,8 +1978,8 @@ static void do_trap_stage2_abort_guest(struct
> cpu_user_regs *regs,
> * with the Stage-2 page table. Walk the Stage-2 PT to check
> * if the entry exists. If it's the case, return to the guest
> */
> - mfn = gfn_to_mfn(current->domain, gaddr_to_gfn(gpa));
> - if ( !mfn_eq(mfn, INVALID_MFN) )
> + if ( p2m_resolve_translation_fault(current->domain,
> + gaddr_to_gfn(gpa)) )
> return;
>
> if ( is_data && try_map_mmio(gaddr_to_gfn(gpa)) )
> --
> 2.11.0
>
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