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Re: [Xen-ia64-devel] One unstablity in fast syscall path

To: Isaku Yamahata <yamahata@xxxxxxxxxxxxx>, "Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>
Subject: Re: [Xen-ia64-devel] One unstablity in fast syscall path
From: Tristan Gingold <Tristan.Gingold@xxxxxxxx>
Date: Wed, 14 Jun 2006 10:20:26 +0200
Cc: xen-ia64-devel <xen-ia64-devel@xxxxxxxxxxxxxxxxxxx>
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Le Mercredi 14 Juin 2006 05:02, Isaku Yamahata a écrit :
> On Tue, Jun 13, 2006 at 01:11:04PM -0700, Magenheimer, Dan (HP Labs Fort 
Collins) wrote:
> > There are two purposes of paravirtualization: one is correctness
> > and the other is performance.  If "fully virtualized" vDSO
> > works properly and there's no impact on performance, it
> > shouldn't be paravirtualized.  If there is a measurable
> > impact, it should be paravirtualized.
> I fully agree with you that paravirtualization for performance
> must be backed by a sort of measurement.
> I have a question on priv_handle_op().
> I changed the function so that xen/ia64 reflects itlb miss to a domain
> when xen/ia64 fails to read a bundle.
> Xen/ia64 reflected dtlb miss before my change.
> Is it correct to reflect dtlb miss?
> I guess you already encountered the same problem
> and gave the consideration on it.
Just a small comment here:
before your patch itlb was useless: it was never read.
This works because Linux assume I space == D space.  But this may be wrong for 
other OS.

It was of course wrong to read the bundle directly in the D space, but for 
sure it was faster than doing manually the translation.
On this point VTi may have a real advantage over paravirtualization.


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