Re: [Qemu-devel] [PATCH v7 3/5] migration: API to clear bits of guest free pages from the dirty bitmap

[Wei Wang <wei.w.wang@intel.com>]

On 06/01/2018 06:06 PM, Peter Xu wrote:
> On Fri, Jun 01, 2018 at 03:36:01PM +0800, Wei Wang wrote:
> > On 06/01/2018 12:00 PM, Peter Xu wrote:
> > > On Tue, Apr 24, 2018 at 02:13:46PM +0800, Wei Wang wrote:
> > > >    /*
> > > > + * This function clears bits of the free pages reported by the caller from the
> > > > + * migration dirty bitmap. @addr is the host address corresponding to the
> > > > + * start of the continuous guest free pages, and @len is the total bytes of
> > > > + * those pages.
> > > > + */
> > > > +void qemu_guest_free_page_hint(void *addr, size_t len)
> > > > +{
> > > > +    RAMBlock *block;
> > > > +    ram_addr_t offset;
> > > > +    size_t used_len, start, npages;
> > > Do we need to check here on whether a migration is in progress?  Since
> > > if not I'm not sure whether this hint still makes any sense any more,
> > > and more importantly it seems to me that block->bmap below at [1] is
> > > only valid during a migration.  So I'm not sure whether QEMU will
> > > crash if this function is called without a running migration.
> > OK. How about just adding comments above to have users noted that this
> > function should be used during migration?
> >
> > If we want to do a sanity check here, I think it would be easier to just
> > check !block->bmap here.
> I think the faster way might be that we check against the migration
> state.

Sounds good. We can do a sanity check:

    MigrationState *s = migrate_get_current();
    if (!migration_is_setup_or_active(s->state))
        return;



> > > > +
> > > > +    for (; len > 0; len -= used_len) {
> > > > +        block = qemu_ram_block_from_host(addr, false, &offset);
> > > > +        if (unlikely(!block)) {
> > > > +            return;
> > > We should never reach here, should we?  Assuming the callers of this
> > > function should always pass in a correct host address. If we are very
> > > sure that the host addr should be valid, could we just assert?
> > Probably not the case, because of the corner case that the memory would be
> > hot unplugged after the free page is reported to QEMU.
> Question: Do we allow to do hot plug/unplug for memory during
> migration?

I think so. From the code, I don't find where it forbids memory hotplug 
during migration.

> > > > +        }
> > > > +
> > > > +        /*
> > > > +         * This handles the case that the RAMBlock is resized after the free
> > > > +         * page hint is reported.
> > > > +         */
> > > > +        if (unlikely(offset > block->used_length)) {
> > > > +            return;
> > > > +        }
> > > > +
> > > > +        if (len <= block->used_length - offset) {
> > > > +            used_len = len;
> > > > +        } else {
> > > > +            used_len = block->used_length - offset;
> > > > +            addr += used_len;
> > > > +        }
> > > > +
> > > > +        start = offset >> TARGET_PAGE_BITS;
> > > > +        npages = used_len >> TARGET_PAGE_BITS;
> > > > +
> > > > +        qemu_mutex_lock(&ram_state->bitmap_mutex);
> > > So now I think I understand the lock can still be meaningful since
> > > this function now can be called outside the migration thread (e.g., in
> > > vcpu thread).  But still it would be nice to mention it somewhere on
> (Actually after read the next patch I think it's in iothread, so I'd
>   better reply with all the series read over next time :)

That's fine actually :) Whether it is called by an iothread or a vcpu 
thread doesn't affect our discussion here.

I think we could just focus on the interfaces here and the usage in live 
migration. I can explain more when needed.

Best,
Wei