linux-kernel - Re: [PATCH -fixes] riscv: Implement flush_cache

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Message-ID: <20230803-stadium-unusable-6cf00e35ec22@wendy>
Date:   Thu, 3 Aug 2023 10:24:48 +0100
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     dylan <dylan@...estech.com>
CC:     Guo Ren <guoren@...nel.org>,
        Alexandre Ghiti <alexghiti@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Björn Töpel <bjorn@...osinc.com>,
        <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -fixes] riscv: Implement flush_cache_vmap()

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@...osinc.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@...osinc.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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