Re: [Xen-ia64-devel] PATCH: cleanup of tlbflush

[Tristan Gingold <Tristan.Gingold@xxxxxxxx>]

Le Mercredi 10 Mai 2006 18:01, Isaku Yamahata a écrit :
> On Wed, May 10, 2006 at 07:38:12PM +0800, Tian, Kevin wrote:
> > >From: Tristan Gingold [mailto:Tristan.Gingold@xxxxxxxx]
> > >Sent: 2006年5月10日 18:47
> > >
> > >> I see your concern about flush efficiency. However we still need set
> > >> necessary mask bits for correctness, right?
> > >
> > >Not yet, because pages are not transfered.
> >
> > It's not specific to page flipping. Simple page sharing also has same
> > problem.
> >
> > >> It would be difficult to track
> > >> exact processors which have footprint about different ungranted
> > >
> > >pages.
> > >
> > >> To track that list may instead pull down performance at other places.
> > >> Then to set domain_dirty_cpumask as ones that domain is currently
> > >> running on, can be a simple/safe way in current stage though
> > >> performance may be affected.
> > >
> > >Unfortunatly performance are so badly affected that using SMP-g is
> > >useless!
> >
> > If correctness becomes an issue, like shared va has footprint on
> > several vcpus, you have to flush tlb on multiple processors or else
> > SMP-g is broken.
> >
> > After more thinking, I think there's no need for flush_tlb_mask to flush
> > both tlb all and vhpt all. Flush_tlb_mask just does as what the name
> > stands for: flushing all related TLBs indicating in the
> > domain_dirty_cpumask. Instead the affected software structures can
> > be always flushed in destroy_grant_host_mapping().
> >
> > For xen/x86, destroy_grant_host_mapping clears affected pte entry in
> > writable page table or the pte entry in shadow page table based on
> > host_addr.
> >
> > For xen/ia64, the vhpt table can be flushed by host_addr too, in
> > destroy_grant_host_mapping. For each requested unmap page, only
> > affected vhpt entry will be flushed and there's no need for full purge.
> >
> > The key point is to pass in the gva address (host_addr) which is
> > previously mapped to granted frame. It's guest's responsibility to record
> > those mapped address and then passed in at unmap request. For
> > example, xen/x86 use pre-allocated virtual address range while xen/ia64
> > uses identity-mapped one. It's current para-driver style and we can
> > trust domain since guest needs to be cooperative or else domain itself
> > is messed instead of xen.
> >
> > Isaku, how about your thought on it?
>
> I don't think that tracking virtual address cause much performance loss.
> At least for vbd.
> The reason is that a underlying block device doesn't need to
> read its data. Then unmapping such a granted page doesn't require
> any flush. (I'm just guessing. The md driver or lvm may read its
> contents to calculate checksum though.)
Some disk device drivers also don't use DMA!

> We can enhance grant table to allow no-read/no-write(dma-only) mapping.

Tristan.

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