Re: [PATCH 1/1] riscv: Implement arch_sync_kernel_mappings() for "preventive" TLB flush

[Alexandre Ghiti]

Hi Dylan,

On 07/08/2023 10:23, Dylan Jhong wrote:
> Since RISC-V is a microarchitecture that allows caching invalid entries in the TLB,
> it is necessary to issue a "preventive" SFENCE.VMA to ensure that each core obtains
> the correct kernel mapping.
>
> The patch implements TLB flushing in arch_sync_kernel_mappings(), ensuring that kernel
> page table mappings created via vmap/vmalloc() are updated before switching MM.
>
> Signed-off-by: Dylan Jhong <dylan@xxxxxxxxxxxxx>
> ---
>   arch/riscv/include/asm/page.h |  2 ++
>   arch/riscv/mm/tlbflush.c      | 12 ++++++++++++
>   2 files changed, 14 insertions(+)
>
> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
> index b55ba20903ec..6c86ab69687e 100644
> --- a/arch/riscv/include/asm/page.h
> +++ b/arch/riscv/include/asm/page.h
> @@ -21,6 +21,8 @@
>   #define HPAGE_MASK              (~(HPAGE_SIZE - 1))
>   #define HUGETLB_PAGE_ORDER      (HPAGE_SHIFT - PAGE_SHIFT)
>   
> +#define ARCH_PAGE_TABLE_SYNC_MASK	PGTBL_PTE_MODIFIED
> +
>   /*
>    * PAGE_OFFSET -- the first address of the first page of memory.
>    * When not using MMU this corresponds to the first free page in
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index 77be59aadc73..d63364948c85 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -149,3 +149,15 @@ void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,
>   	__flush_tlb_range(vma->vm_mm, start, end - start, PMD_SIZE);
>   }
>   #endif
> +
> +/*
> + * Since RISC-V is a microarchitecture that allows caching invalid entries in the TLB,
> + * it is necessary to issue a "preventive" SFENCE.VMA to ensure that each core obtains
> + * the correct kernel mapping. arch_sync_kernel_mappings() will ensure that kernel
> + * page table mappings created via vmap/vmalloc() are updated before switching MM.
> + */
> +void arch_sync_kernel_mappings(unsigned long start, unsigned long end)
> +{
> +	if (start < VMALLOC_END && end > VMALLOC_START)


This test is too restrictive, it should catch the range [MODULES_VADDR;  
MODULES_END[ too, sorry I did not notice that at first.


> +		flush_tlb_all();
> +}
> \ No newline at end of file


I have to admit that I *think* both your patch and mine are wrong: one 
of the problem that led to the removal of vmalloc_fault() is the 
possibility for tracing functions to actually allocate vmalloc regions 
in the vmalloc page fault path, which could give rise to nested 
exceptions (see 
https://lore.kernel.org/lkml/20200508144043.13893-1-joro@xxxxxxxxxx/).

Here, everytime we allocate a vmalloc region, we send an IPI. If a 
vmalloc allocation happens in this path (if it is traced for example), 
it will give rise to an IPI...and so on.

So I came to the conclusion that the only way to actually fix this issue 
is by resolving the vmalloc faults very early in the page fault path (by 
emitting a sfence.vma on uarch that cache invalid entries), before the 
kernel stack is even accessed. That's the best solution since it would 
completely remove all the preventive sfence.vma in 
flush_cache_vmap()/arch_sync_kernel_mappings(), we would rely on 
faulting which I assume should not happen a lot (?).

I'm implementing this solution, but I'm pretty sure it won't be ready 
for 6.5. In the meantime, we need either your patch or mine to fix your 
issue...