I don't think it is sensible that any hardware would actually cache invalid entries.
From my research https://arxiv.org/pdf/1905.06825 (will appear in CARRV 2019), existing Linux code already emits too many SFENCE.VMAs (94% of all flushes) that are completely unnecessary for a hardware that does not cache invalid entries. Adding a couple of more would definitely not good, considering that TLB flush could be expensive for some microarchitectures.
I think the spec should be fixed, recommending against invalid entries to be cached, and then we can do things similar to other architectures, i.e. use flush_tlb_fix_spurious_fault instead.
-----Original Message----- From: linux-riscv linux-riscv-bounces@lists.infradead.org On Behalf Of Paul Walmsley Sent: Monday, June 17, 2019 11:33 To: ShihPo Hung shihpo.hung@sifive.com Cc: linux-riscv@lists.infradead.org; Palmer Dabbelt palmer@sifive.com; Albert Ou aou@eecs.berkeley.edu; stable@vger.kernel.org Subject: Re: [PATCH v2] riscv: mm: synchronize MMU after pte change
Hi ShihPo,
On Mon, 17 Jun 2019, ShihPo Hung wrote:
Because RISC-V compliant implementations can cache invalid entries in TLB, an SFENCE.VMA is necessary after changes to the page table. This patch adds an SFENCE.vma for the vmalloc_fault path.
Signed-off-by: ShihPo Hung shihpo.hung@sifive.com Cc: Palmer Dabbelt palmer@sifive.com Cc: Albert Ou aou@eecs.berkeley.edu Cc: Paul Walmsley paul.walmsley@sifive.com Cc: linux-riscv@lists.infradead.org Cc: stable@vger.kernel.org
Looks like something in your patch flow converted tabs to whitespace, so the patch doesn't apply. Then, with the tabs fixed, the comment text exceeds 80 columns.
I've fixed those issues by hand for this patch (revised patch below, as queued for v5.2-rc), but could you please fix these issues for future patches? Running checkpatch.pl --strict should help identify these issues before posting.
thanks,
- Paul
From: ShihPo Hung shihpo.hung@sifive.com Date: Mon, 17 Jun 2019 12:26:17 +0800 Subject: [PATCH] [PATCH v2] riscv: mm: synchronize MMU after pte change
Because RISC-V compliant implementations can cache invalid entries in TLB, an SFENCE.VMA is necessary after changes to the page table. This patch adds an SFENCE.vma for the vmalloc_fault path.
Signed-off-by: ShihPo Hung shihpo.hung@sifive.com [paul.walmsley@sifive.com: reversed tab->whitespace conversion, wrapped comment lines] Signed-off-by: Paul Walmsley paul.walmsley@sifive.com Cc: Palmer Dabbelt palmer@sifive.com Cc: Albert Ou aou@eecs.berkeley.edu Cc: Paul Walmsley paul.walmsley@sifive.com Cc: linux-riscv@lists.infradead.org Cc: stable@vger.kernel.org
arch/riscv/mm/fault.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)
diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c index cec8be9e2d6a..5b72e60c5a6b 100644 --- a/arch/riscv/mm/fault.c +++ b/arch/riscv/mm/fault.c @@ -29,6 +29,7 @@
#include <asm/pgalloc.h> #include <asm/ptrace.h> +#include <asm/tlbflush.h>
/*
- This routine handles page faults. It determines the address and the
@@ -278,6 +279,18 @@ asmlinkage void do_page_fault(struct pt_regs *regs) pte_k = pte_offset_kernel(pmd_k, addr); if (!pte_present(*pte_k)) goto no_context;
/** The kernel assumes that TLBs don't cache invalid* entries, but in RISC-V, SFENCE.VMA specifies an* ordering constraint, not a cache flush; it is* necessary even after writing invalid entries.* Relying on flush_tlb_fix_spurious_fault would* suffice, but the extra traps reduce* performance. So, eagerly SFENCE.VMA.*/local_flush_tlb_page(addr);- return; }
}
2.20.1
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