Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

Jason Wang
2019-12-05T06:51:33Z

On 2019/12/5 上午3:52, Peter Xu wrote:
> On Wed, Dec 04, 2019 at 12:04:53PM +0100, Paolo Bonzini wrote:
>> On 04/12/19 11:38, Jason Wang wrote:
>>>> +    entry = &ring->dirty_gfns[ring->dirty_index & (ring->size - 1)];
>>>> +    entry->slot = slot;
>>>> +    entry->offset = offset;
>>>
>>> Haven't gone through the whole series, sorry if it was a silly question
>>> but I wonder things like this will suffer from similar issue on
>>> virtually tagged archs as mentioned in [1].
>> There is no new infrastructure to track the dirty pages---it's just a
>> different way to pass them to userspace.
>>
>>> Is this better to allocate the ring from userspace and set to KVM
>>> instead? Then we can use copy_to/from_user() friends (a little bit slow
>>> on recent CPUs).
>> Yeah, I don't think that would be better than mmap.
> Yeah I agree, because I didn't see how copy_to/from_user() helped to
> do icache/dcache flushings...

It looks to me one advantage is that exact the same VA is used by both
userspace and kernel so there will be no alias.

Thanks

>
> Some context here: Jason raised this question offlist first on whether
> we should also need these flush_dcache_cache() helpers for operations
> like kvm dirty ring accesses. I feel like it should, however I've got
> two other questions, on:
>
> - if we need to do flush_dcache_page() on kernel modified pages
> (assuming the same page has mapped to userspace), then why don't
> we need flush_cache_page() too on the page, where
> flush_cache_page() is defined not-a-nop on those archs?
>
> - assuming an arch has not-a-nop impl for flush_[d]cache_page(),
> would atomic operations like cmpxchg really work for them
> (assuming that ISAs like cmpxchg should depend on cache
> consistency).
>
> Sorry I think these are for sure a bit out of topic for kvm dirty ring
> patchset, but since we're at it, I'm raising the questions up in case
> there're answers..
>
> Thanks,
>