Re: [PATCH] Insert SFENCE.VMA in function set_pte_at for RISCV

[Alex Ghiti <alex@ghiti.fr>]

Le 3/17/21 à 10:10 PM, Jiuyang Liu a écrit :
> Thanks for the review!
> 
> I see, after skimming related codes, and implementation of other architecture,
> I also agree this method is too heavy to implement. And there is a potential
> bug, that my patch may introduce two SFENCE.VMA in the related codes:
> flush at set_pte_at and also flush in the upper level of the calling stack.
> 
> My two cents is that the original description in spec is a little
> misleading to the
> software side, spec requires each set_pte inserting SFENCE.VMA together,
> while the kernel chooses to maintain set_pte and flush_tlb separately.
> 
> So I think I should add a patch to fix my bug specifically, and
> provide this trunk
> as an inline function to flush tlb after modification to a pte.
> 
>> if (pte_present(pteval)) {
>>          if (pte_leaf(pteval)) {
>>                  local_flush_tlb_page(addr);
>>          } else {
>>                  if (pte_global(pteval))
>>                          local_flush_tlb_all();
>>                  else
>>                          local_flush_tlb_asid();
>>
>>         }
>> }
> 
> My next patch will become two patches:
> 1. add flush_tlb related codes according to spec(also flush global tlb
> via sbi call if G bit is on)
> 2. add a bug fix for my stack by adding flush in the flush_cache_vmap.
> 
> Does this approach sound reasonable?

Ok for me, please take a look at flush_cache_vunmap too as I think we 
need to do the same thing here.

Thanks,

Alex

> 
> Regards,
> Jiuyang
> 
> On Tue, 16 Mar 2021 at 09:17 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>> We're trying to avoid this sort of thing, instead relying on the generic kernel
>> functionality to batch up page table modifications before we issue the fences.
>> If you're seeing some specific issue then I'd be happy to try and sort out a
>> fix for it, but this is a bit heavy-handed to use as anything but a last
>> resort.
> On Tue, Mar 16, 2021 at 10:03 PM Andrew Waterman
> <waterman@eecs.berkeley.edu> wrote:
>>
>> On Tue, Mar 16, 2021 at 5:05 AM Alex Ghiti <alex@ghiti.fr> wrote:
>>>
>>> Le 3/16/21 à 4:40 AM, Anup Patel a écrit :
>>>> On Tue, Mar 16, 2021 at 1:59 PM Andrew Waterman
>>>> <waterman@eecs.berkeley.edu> wrote:
>>>>>
>>>>> On Tue, Mar 16, 2021 at 12:32 AM Anup Patel <anup@brainfault.org> wrote:
>>>>>>
>>>>>> On Tue, Mar 16, 2021 at 12:27 PM Jiuyang Liu <liu@jiuyang.me> wrote:
>>>>>>>
>>>>>>>> As per my understanding, we don't need to explicitly invalidate local TLB
>>>>>>>> in set_pte() or set_pet_at() because generic Linux page table management
>>>>>>>> (<linux>/mm/*) will call the appropriate flush_tlb_xyz() function after page
>>>>>>>> table updates.
>>>>>>>
>>>>>>> I witnessed this bug in our micro-architecture: set_pte instruction is
>>>>>>> still in the store buffer, no functions are inserting SFENCE.VMA in
>>>>>>> the stack below, so TLB cannot witness this modification.
>>>>>>> Here is my call stack:
>>>>>>> set_pte
>>>>>>> set_pte_at
>>>>>>> map_vm_area
>>>>>>> __vmalloc_area_node
>>>>>>> __vmalloc_node_range
>>>>>>> __vmalloc_node
>>>>>>> __vmalloc_node_flags
>>>>>>> vzalloc
>>>>>>> n_tty_open
>>>>>>>
>>>
>>> I don't find this call stack, what I find is (the other way around):
>>>
>>> n_tty_open
>>> vzalloc
>>> __vmalloc_node
>>> __vmalloc_node_range
>>> __vmalloc_area_node
>>> map_kernel_range
>>> -> map_kernel_range_noflush
>>>      flush_cache_vmap
>>>
>>> Which leads to the fact that we don't have flush_cache_vmap callback
>>> implemented: shouldn't we add the sfence.vma here ? Powerpc does
>>> something similar with "ptesync" (see below) instruction that seems to
>>> do the same as sfence.vma.
>>
>> I was thinking the same thing, but I hadn't yet wrapped my head around
>> the fact that most architectures don't have something similar.  I'm OK
>> with following PPC's lead if it appears to be a correct bug fix :)
>>
>>>
>>>
>>> ptesync: "The ptesync instruction after the Store instruction ensures
>>> that all searches of the Page Table that are performed after the ptesync
>>> instruction completes will use the value stored"
>>>
>>>>>>> I think this is an architecture specific code, so <linux>/mm/* should
>>>>>>> not be modified.
>>>>>>> And spec requires SFENCE.VMA to be inserted on each modification to
>>>>>>> TLB. So I added code here.
>>>>>>
>>>>>> The generic linux/mm/* already calls the appropriate tlb_flush_xyz()
>>>>>> function defined in arch/riscv/include/asm/tlbflush.h
>>>>>>
>>>>>> Better to have a write-barrier in set_pte().
>>>>>>
>>>>>>>
>>>>>>>> Also, just local TLB flush is generally not sufficient because
>>>>>>>> a lot of page tables will be used across on multiple HARTs.
>>>>>>>
>>>>>>> Yes, this is the biggest issue, in RISC-V Volume 2, Privileged Spec v.
>>>>>>> 20190608 page 67 gave a solution:
>>>>>>
>>>>>> This is not an issue with RISC-V privilege spec rather it is more about
>>>>>> placing RISC-V fences at right locations.
>>>>>>
>>>>>>> Consequently, other harts must be notified separately when the
>>>>>>> memory-management data structures have been modified. One approach is
>>>>>>> to use
>>>>>>> 1) a local data fence to ensure local writes are visible globally,
>>>>>>> then 2) an interprocessor interrupt to the other thread,
>>>>>>> then 3) a local SFENCE.VMA in the interrupt handler of the remote thread,
>>>>>>> and finally 4) signal back to originating thread that operation is
>>>>>>> complete. This is, of course, the RISC-V analog to a TLB shootdown.
>>>>>>
>>>>>> I would suggest trying approach#1.
>>>>>>
>>>>>> You can include "asm/barrier.h" here and use wmb() or __smp_wmb()
>>>>>> in-place of local TLB flush.
>>>>>
>>>>> wmb() doesn't suffice to order older stores before younger page-table
>>>>> walks, so that might hide the problem without actually fixing it.
>>>>
>>>> If we assume page-table walks as reads then mb() might be more
>>>> suitable in this case ??
>>>>
>>>> ARM64 also has an explicit barrier in set_pte() implementation. They are
>>>> doing "dsb(ishst); isb()" which is an inner-shareable store barrier followed
>>>> by an instruction barrier.
>>>>
>>>>>
>>>>> Based upon Jiuyang's description, it does sound plausible that we are
>>>>> missing an SFENCE.VMA (or TLB shootdown) somewhere.  But I don't
>>>>> understand the situation well enough to know where that might be, or
>>>>> what the best fix is.
>>>>
>>>> Yes, I agree but set_pte() doesn't seem to be the right place for TLB
>>>> shootdown based on set_pte() implementations of other architectures.
>>>
>>> I agree as "flushing" the TLB after every set_pte() would be very
>>> costly, it's better to do it once at the end of the all the updates:
>>> like in flush_cache_vmap :)
>>>
>>> Alex
>>>
>>>>
>>>> Regards,
>>>> Anup
>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>> In general, this patch didn't handle the G bit in PTE, kernel trap it
>>>>>>> to sbi_remote_sfence_vma. do you think I should use flush_tlb_all?
>>>>>>>
>>>>>>> Jiuyang
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> arch/arm/mm/mmu.c
>>>>>>> void set_pte_at(struct mm_struct *mm, unsigned long addr,
>>>>>>>                                 pte_t *ptep, pte_t pteval)
>>>>>>> {
>>>>>>>           unsigned long ext = 0;
>>>>>>>
>>>>>>>           if (addr < TASK_SIZE && pte_valid_user(pteval)) {
>>>>>>>                   if (!pte_special(pteval))
>>>>>>>                           __sync_icache_dcache(pteval);
>>>>>>>                   ext |= PTE_EXT_NG;
>>>>>>>           }
>>>>>>>
>>>>>>>           set_pte_ext(ptep, pteval, ext);
>>>>>>> }
>>>>>>>
>>>>>>> arch/mips/include/asm/pgtable.h
>>>>>>> static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>>>>>>>                                 pte_t *ptep, pte_t pteval)
>>>>>>> {
>>>>>>>
>>>>>>>           if (!pte_present(pteval))
>>>>>>>                   goto cache_sync_done;
>>>>>>>
>>>>>>>           if (pte_present(*ptep) && (pte_pfn(*ptep) == pte_pfn(pteval)))
>>>>>>>                   goto cache_sync_done;
>>>>>>>
>>>>>>>           __update_cache(addr, pteval);
>>>>>>> cache_sync_done:
>>>>>>>           set_pte(ptep, pteval);
>>>>>>> }
>>>>>>>
>>>>>>>
>>>>>>> Also, just local TLB flush is generally not sufficient because
>>>>>>>> a lot of page tables will be used accross on multiple HARTs.
>>>>>>>
>>>>>>>
>>>>>>> On Tue, Mar 16, 2021 at 5:05 AM Anup Patel <anup@brainfault.org> wrote:
>>>>>>>>
>>>>>>>> +Alex
>>>>>>>>
>>>>>>>> On Tue, Mar 16, 2021 at 9:20 AM Jiuyang Liu <liu@jiuyang.me> wrote:
>>>>>>>>>
>>>>>>>>> This patch inserts SFENCE.VMA after modifying PTE based on RISC-V
>>>>>>>>> specification.
>>>>>>>>>
>>>>>>>>> arch/riscv/include/asm/pgtable.h:
>>>>>>>>> 1. implement pte_user, pte_global and pte_leaf to check correspond
>>>>>>>>> attribute of a pte_t.
>>>>>>>>
>>>>>>>> Adding pte_user(), pte_global(), and pte_leaf() is fine.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> 2. insert SFENCE.VMA in set_pte_at based on RISC-V Volume 2, Privileged
>>>>>>>>> Spec v. 20190608 page 66 and 67:
>>>>>>>>> If software modifies a non-leaf PTE, it should execute SFENCE.VMA with
>>>>>>>>> rs1=x0. If any PTE along the traversal path had its G bit set, rs2 must
>>>>>>>>> be x0; otherwise, rs2 should be set to the ASID for which the
>>>>>>>>> translation is being modified.
>>>>>>>>> If software modifies a leaf PTE, it should execute SFENCE.VMA with rs1
>>>>>>>>> set to a virtual address within the page. If any PTE along the traversal
>>>>>>>>> path had its G bit set, rs2 must be x0; otherwise, rs2 should be set to
>>>>>>>>> the ASID for which the translation is being modified.
>>>>>>>>>
>>>>>>>>> arch/riscv/include/asm/tlbflush.h:
>>>>>>>>> 1. implement get_current_asid to get current program asid.
>>>>>>>>> 2. implement local_flush_tlb_asid to flush tlb with asid.
>>>>>>>>
>>>>>>>> As per my understanding, we don't need to explicitly invalidate local TLB
>>>>>>>> in set_pte() or set_pet_at() because generic Linux page table management
>>>>>>>> (<linux>/mm/*) will call the appropriate flush_tlb_xyz() function after page
>>>>>>>> table updates. Also, just local TLB flush is generally not sufficient because
>>>>>>>> a lot of page tables will be used accross on multiple HARTs.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Signed-off-by: Jiuyang Liu <liu@jiuyang.me>
>>>>>>>>> ---
>>>>>>>>>    arch/riscv/include/asm/pgtable.h  | 27 +++++++++++++++++++++++++++
>>>>>>>>>    arch/riscv/include/asm/tlbflush.h | 12 ++++++++++++
>>>>>>>>>    2 files changed, 39 insertions(+)
>>>>>>>>>
>>>>>>>>> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
>>>>>>>>> index ebf817c1bdf4..5a47c60372c1 100644
>>>>>>>>> --- a/arch/riscv/include/asm/pgtable.h
>>>>>>>>> +++ b/arch/riscv/include/asm/pgtable.h
>>>>>>>>> @@ -222,6 +222,16 @@ static inline int pte_write(pte_t pte)
>>>>>>>>>           return pte_val(pte) & _PAGE_WRITE;
>>>>>>>>>    }
>>>>>>>>>
>>>>>>>>> +static inline int pte_user(pte_t pte)
>>>>>>>>> +{
>>>>>>>>> +       return pte_val(pte) & _PAGE_USER;
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>> +static inline int pte_global(pte_t pte)
>>>>>>>>> +{
>>>>>>>>> +       return pte_val(pte) & _PAGE_GLOBAL;
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>>    static inline int pte_exec(pte_t pte)
>>>>>>>>>    {
>>>>>>>>>           return pte_val(pte) & _PAGE_EXEC;
>>>>>>>>> @@ -248,6 +258,11 @@ static inline int pte_special(pte_t pte)
>>>>>>>>>           return pte_val(pte) & _PAGE_SPECIAL;
>>>>>>>>>    }
>>>>>>>>>
>>>>>>>>> +static inline int pte_leaf(pte_t pte)
>>>>>>>>> +{
>>>>>>>>> +       return pte_val(pte) & (_PAGE_READ | _PAGE_WRITE | _PAGE_EXEC);
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>>    /* static inline pte_t pte_rdprotect(pte_t pte) */
>>>>>>>>>
>>>>>>>>>    static inline pte_t pte_wrprotect(pte_t pte)
>>>>>>>>> @@ -358,6 +373,18 @@ static inline void set_pte_at(struct mm_struct *mm,
>>>>>>>>>                   flush_icache_pte(pteval);
>>>>>>>>>
>>>>>>>>>           set_pte(ptep, pteval);
>>>>>>>>> +
>>>>>>>>> +       if (pte_present(pteval)) {
>>>>>>>>> +               if (pte_leaf(pteval)) {
>>>>>>>>> +                       local_flush_tlb_page(addr);
>>>>>>>>> +               } else {
>>>>>>>>> +                       if (pte_global(pteval))
>>>>>>>>> +                               local_flush_tlb_all();
>>>>>>>>> +                       else
>>>>>>>>> +                               local_flush_tlb_asid();
>>>>>>>>> +
>>>>>>>>> +               }
>>>>>>>>> +       }
>>>>>>>>>    }
>>>>>>>>>
>>>>>>>>>    static inline void pte_clear(struct mm_struct *mm,
>>>>>>>>> diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
>>>>>>>>> index 394cfbccdcd9..1f9b62b3670b 100644
>>>>>>>>> --- a/arch/riscv/include/asm/tlbflush.h
>>>>>>>>> +++ b/arch/riscv/include/asm/tlbflush.h
>>>>>>>>> @@ -21,6 +21,18 @@ static inline void local_flush_tlb_page(unsigned long addr)
>>>>>>>>>    {
>>>>>>>>>           __asm__ __volatile__ ("sfence.vma %0" : : "r" (addr) : "memory");
>>>>>>>>>    }
>>>>>>>>> +
>>>>>>>>> +static inline unsigned long get_current_asid(void)
>>>>>>>>> +{
>>>>>>>>> +       return (csr_read(CSR_SATP) >> SATP_ASID_SHIFT) & SATP_ASID_MASK;
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>> +static inline void local_flush_tlb_asid(void)
>>>>>>>>> +{
>>>>>>>>> +       unsigned long asid = get_current_asid();
>>>>>>>>> +       __asm__ __volatile__ ("sfence.vma x0, %0" : : "r" (asid) : "memory");
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>>    #else /* CONFIG_MMU */
>>>>>>>>>    #define local_flush_tlb_all()                  do { } while (0)
>>>>>>>>>    #define local_flush_tlb_page(addr)             do { } while (0)
>>>>>>>>> --
>>>>>>>>> 2.30.2
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> linux-riscv mailing list
>>>>>>>>> linux-riscv@lists.infradead.org
>>>>>>>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>>>>>>>>
>>>>>>>> Regards,
>>>>>>>> Anup
>>>>>>
>>>>>> Regards,
>>>>>> Anup
>>>>
>>>> _______________________________________________
>>>> linux-riscv mailing list
>>>> linux-riscv@lists.infradead.org
>>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>>>>
> 
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