Re: [PATCH RFC/RFT 2/4] riscv: Add a runtime detection of invalid TLB entries caching

[Alexandre Ghiti]

On Thu, Dec 7, 2023 at 4:55 PM Christophe Leroy
<christophe.leroy@xxxxxxxxxx> wrote:
>
>
>
> Le 07/12/2023 à 16:03, Alexandre Ghiti a écrit :
> > This mechanism allows to completely bypass the sfence.vma introduced by
> > the previous commit for uarchs that do not cache invalid TLB entries.
> >
> > Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> > ---
> >   arch/riscv/mm/init.c | 124 +++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 124 insertions(+)
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index 379403de6c6f..2e854613740c 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -56,6 +56,8 @@ bool pgtable_l5_enabled = IS_ENABLED(CONFIG_64BIT) && !IS_ENABLED(CONFIG_XIP_KER
> >   EXPORT_SYMBOL(pgtable_l4_enabled);
> >   EXPORT_SYMBOL(pgtable_l5_enabled);
> >
> > +bool tlb_caching_invalid_entries;
> > +
> >   phys_addr_t phys_ram_base __ro_after_init;
> >   EXPORT_SYMBOL(phys_ram_base);
> >
> > @@ -750,6 +752,18 @@ static void __init disable_pgtable_l4(void)
> >       satp_mode = SATP_MODE_39;
> >   }
> >
> > +static void __init enable_pgtable_l5(void)
> > +{
> > +     pgtable_l5_enabled = true;
> > +     satp_mode = SATP_MODE_57;
> > +}
> > +
> > +static void __init enable_pgtable_l4(void)
> > +{
> > +     pgtable_l4_enabled = true;
> > +     satp_mode = SATP_MODE_48;
> > +}
> > +
> >   static int __init print_no4lvl(char *p)
> >   {
> >       pr_info("Disabled 4-level and 5-level paging");
> > @@ -826,6 +840,112 @@ static __init void set_satp_mode(uintptr_t dtb_pa)
> >       memset(early_pud, 0, PAGE_SIZE);
> >       memset(early_pmd, 0, PAGE_SIZE);
> >   }
> > +
> > +/* Determine at runtime if the uarch caches invalid TLB entries */
> > +static __init void set_tlb_caching_invalid_entries(void)
> > +{
> > +#define NR_RETRIES_CACHING_INVALID_ENTRIES   50
>
> Looks odd to have macros nested in the middle of a function.
>
> > +     uintptr_t set_tlb_caching_invalid_entries_pmd = ((unsigned long)set_tlb_caching_invalid_entries) & PMD_MASK;
> > +     // TODO the test_addr as defined below could go into another pud...
> > +     uintptr_t test_addr = set_tlb_caching_invalid_entries_pmd + 2 * PMD_SIZE;
> > +     pmd_t valid_pmd;
> > +     u64 satp;
> > +     int i = 0;
> > +
> > +     /* To ease the page table creation */
> > +     disable_pgtable_l5();
> > +     disable_pgtable_l4();
> > +
> > +     /* Establish a mapping for set_tlb_caching_invalid_entries() in sv39 */
> > +     create_pgd_mapping(early_pg_dir,
> > +                        set_tlb_caching_invalid_entries_pmd,
> > +                        (uintptr_t)early_pmd,
> > +                        PGDIR_SIZE, PAGE_TABLE);
> > +
> > +     /* Handle the case where set_tlb_caching_invalid_entries straddles 2 PMDs */
> > +     create_pmd_mapping(early_pmd,
> > +                        set_tlb_caching_invalid_entries_pmd,
> > +                        set_tlb_caching_invalid_entries_pmd,
> > +                        PMD_SIZE, PAGE_KERNEL_EXEC);
> > +     create_pmd_mapping(early_pmd,
> > +                        set_tlb_caching_invalid_entries_pmd + PMD_SIZE,
> > +                        set_tlb_caching_invalid_entries_pmd + PMD_SIZE,
> > +                        PMD_SIZE, PAGE_KERNEL_EXEC);
> > +
> > +     /* Establish an invalid mapping */
> > +     create_pmd_mapping(early_pmd, test_addr, 0, PMD_SIZE, __pgprot(0));
> > +
> > +     /* Precompute the valid pmd here because the mapping for pfn_pmd() won't exist */
> > +     valid_pmd = pfn_pmd(PFN_DOWN(set_tlb_caching_invalid_entries_pmd), PAGE_KERNEL);
> > +
> > +     local_flush_tlb_all();
> > +     satp = PFN_DOWN((uintptr_t)&early_pg_dir) | SATP_MODE_39;
> > +     csr_write(CSR_SATP, satp);
> > +
> > +     /*
> > +      * Set stvec to after the trapping access, access this invalid mapping
> > +      * and legitimately trap
> > +      */
> > +     // TODO: Should I save the previous stvec?
> > +#define ASM_STR(x)   __ASM_STR(x)
>
> Looks odd to have macros nested in the middle of a function.
>
>
> > +     asm volatile(
> > +             "la a0, 1f                              \n"
> > +             "csrw " ASM_STR(CSR_TVEC) ", a0         \n"
> > +             "ld a0, 0(%0)                           \n"
> > +             ".align 2                               \n"
> > +             "1:                                     \n"
> > +             :
> > +             : "r" (test_addr)
> > +             : "a0"
> > +     );
> > +
> > +     /* Now establish a valid mapping to check if the invalid one is cached */
> > +     early_pmd[pmd_index(test_addr)] = valid_pmd;
> > +
> > +     /*
> > +      * Access the valid mapping multiple times: indeed, we can't use
> > +      * sfence.vma as a barrier to make sure the cpu did not reorder accesses
> > +      * so we may trap even if the uarch does not cache invalid entries. By
> > +      * trying a few times, we make sure that those uarchs will see the right
> > +      * mapping at some point.
> > +      */
> > +
> > +     i = NR_RETRIES_CACHING_INVALID_ENTRIES;
> > +
> > +#define ASM_STR(x)   __ASM_STR(x)
>
> Deplicate define ?
>
> > +     asm_volatile_goto(
> > +             "la a0, 1f                                      \n"
> > +             "csrw " ASM_STR(CSR_TVEC) ", a0                 \n"
> > +             ".align 2                                       \n"
> > +             "1:                                             \n"
> > +             "addi %0, %0, -1                                \n"
> > +             "blt %0, zero, %l[caching_invalid_entries]      \n"
> > +             "ld a0, 0(%1)                                   \n"
> > +             :
> > +             : "r" (i), "r" (test_addr)
> > +             : "a0"
> > +             : caching_invalid_entries
> > +     );
> > +
> > +     csr_write(CSR_SATP, 0ULL);
> > +     local_flush_tlb_all();
> > +
> > +     /* If we don't trap, the uarch does not cache invalid entries! */
> > +     tlb_caching_invalid_entries = false;
> > +     goto clean;
> > +
> > +caching_invalid_entries:
> > +     csr_write(CSR_SATP, 0ULL);
> > +     local_flush_tlb_all();
> > +
> > +     tlb_caching_invalid_entries = true;
> > +clean:
> > +     memset(early_pg_dir, 0, PAGE_SIZE);
> > +     memset(early_pmd, 0, PAGE_SIZE);
>
> Use clear_page() instead ?
>
> > +
> > +     enable_pgtable_l4();
> > +     enable_pgtable_l5();
> > +}
> >   #endif
> >
> >   /*
> > @@ -1072,6 +1192,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> >   #endif
> >
> >   #if defined(CONFIG_64BIT) && !defined(CONFIG_XIP_KERNEL)
> > +     set_tlb_caching_invalid_entries();
> >       set_satp_mode(dtb_pa);
> >   #endif
> >
> > @@ -1322,6 +1443,9 @@ static void __init setup_vm_final(void)
> >       local_flush_tlb_all();
> >
> >       pt_ops_set_late();
> > +
> > +     pr_info("uarch caches invalid entries: %s",
> > +             tlb_caching_invalid_entries ? "yes" : "no");
> >   }
> >   #else
> >   asmlinkage void __init setup_vm(uintptr_t dtb_pa)

I left this patch so that people can easily test this without knowing
what their uarch is actually doing, but it will very likely be dropped
as a new extension has just been proposed for that.

Thanks anyway, I should have been more clear in the patch title,

Alex