Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

[Jason Wang <jasowang@redhat.com>]

On 2019/3/11 下午9:43, Andrea Arcangeli wrote:
> On Mon, Mar 11, 2019 at 08:48:37AM -0400, Michael S. Tsirkin wrote:
>> Using copyXuser is better I guess.
> It certainly would be faster there, but I don't think it's needed if
> that would be the only use case left that justifies supporting two
> different models. On small 32bit systems with little RAM kmap won't
> perform measurably different on 32bit or 64bit systems. If the 32bit
> host has a lot of ram it all gets slow anyway at accessing RAM above
> the direct mapping, if compared to 64bit host kernels, it's not just
> an issue for vhost + mmu notifier + kmap and the best way to optimize
> things is to run 64bit host kernels.
>
> Like Christoph pointed out, the main use case for retaining the
> copy-user model would be CPUs with virtually indexed not physically
> tagged data caches (they'll still suffer from the spectre-v1 fix,
> although I exclude they have to suffer the SMAP
> slowdown/feature). Those may require some additional flushing than the
> current copy-user model requires.
>
> As a rule of thumb any arch where copy_user_page doesn't define as
> copy_page will require some additional cache flushing after the
> kmap. Supposedly with vmap, the vmap layer should have taken care of
> that (I didn't verify that yet).


vmap_page_range()/free_unmap_vmap_area() will call 
fluch_cache_vmap()/flush_cache_vunmap(). So vmap layer should be ok.

Thanks


>
> There are some accessories like copy_to_user_page()
> copy_from_user_page() that could work and obviously defines to raw
> memcpy on x86 (the main cons is they don't provide word granular
> access) and at least on sparc they're tailored to ptrace assumptions
> so then we'd need to evaluate what happens if this is used outside of
> ptrace context. kmap has been used generally either to access whole
> pages (i.e. copy_user_page), so ptrace may actually be the only use
> case with subpage granularity access.
>
> #define copy_to_user_page(vma, page, vaddr, dst, src, len)		\
> 	do {								\
> 		flush_cache_page(vma, vaddr, page_to_pfn(page));	\
> 		memcpy(dst, src, len);					\
> 		flush_ptrace_access(vma, page, vaddr, src, len, 0);	\
> 	} while (0)
>
> So I wouldn't rule out the need for a dual model, until we solve how
> to run this stable on non-x86 arches with not physically tagged
> caches.
>
> Thanks,
> Andrea