Messages in this thread | | | Date | Thu, 3 Aug 2023 10:24:48 +0100 | From | Conor Dooley <> | Subject | Re: [PATCH -fixes] riscv: Implement flush_cache_vmap() |
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On Thu, Aug 03, 2023 at 05:14:15PM +0800, dylan wrote: > On Sun, Jul 30, 2023 at 01:08:17AM -0400, Guo Ren wrote: > > On Tue, Jul 25, 2023 at 9:22 AM Alexandre Ghiti <alexghiti@rivosinc.com> wrote: > > > > > > The RISC-V kernel needs a sfence.vma after a page table modification: we > > > used to rely on the vmalloc fault handling to emit an sfence.vma, but > > > commit 7d3332be011e ("riscv: mm: Pre-allocate PGD entries for > > > vmalloc/modules area") got rid of this path for 64-bit kernels, so now we > > > need to explicitly emit a sfence.vma in flush_cache_vmap(). > > > > > > Note that we don't need to implement flush_cache_vunmap() as the generic > > > code should emit a flush tlb after unmapping a vmalloc region. > > > > > > Fixes: 7d3332be011e ("riscv: mm: Pre-allocate PGD entries for vmalloc/modules area") > > > Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com> > > > --- > > > arch/riscv/include/asm/cacheflush.h | 4 ++++ > > > 1 file changed, 4 insertions(+) > > > > > > diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h > > > index 8091b8bf4883..b93ffddf8a61 100644 > > > --- a/arch/riscv/include/asm/cacheflush.h > > > +++ b/arch/riscv/include/asm/cacheflush.h > > > @@ -37,6 +37,10 @@ static inline void flush_dcache_page(struct page *page) > > > #define flush_icache_user_page(vma, pg, addr, len) \ > > > flush_icache_mm(vma->vm_mm, 0) > > > > > > +#ifdef CONFIG_64BIT > > > +#define flush_cache_vmap(start, end) flush_tlb_kernel_range(start, end) > > Sorry, I couldn't agree with the above in a PIPT cache machine. It's > > not worth for. > > > > It would reduce the performance of vmap_pages_range, > > ioremap_page_range ... API, which may cause some drivers' performance > > issues when they install/uninstall memory frequently. > > > > Hi All, > > I think functional correctness should be more important than system performance > in this case. The "preventive" SFENCE.VMA became necessary due to the RISC-V > specification allowing invalidation entries to be cached in the TLB.
We are at -rc4 and this stuff is broken. Taking the bigger hammer, which can be reverted later when a more targeted fix shows up, to make sure that v6.5 doesn't end up broken, sounds rather prudent. Otherwise, the original commit should probably be reverted.
> The problem[1] we are currently encountering is caused by not updating the TLB > after the page table is created, and the solution to this problem can only be > solved by updating the TLB immediately after the page table is created. > > There are currently two possible approaches to flush TLB: > 1. Flush TLB in flush_cache_vmap() > 2. Flush TLB in arch_sync_kernel_mappings() > > But I'm not quite sure if it's a good idea to operate on the TLB inside flush_cache_vmap(). > The name of this function indicates that it should be related to cache operations, maybe > it would be more appropriate to do TLB flush in arch_sync_kernel_mappings()? > > [1]: http://lists.infradead.org/pipermail/linux-riscv/2023-August/037503.html
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