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Re: [Xen-devel] GPU passthrough performance regression in >4GB vms due to XSA-60 changes

On 05/19/2014 05:24 PM, Jan Beulich wrote:
On 19.05.14 at 16:20, <tomasz.wroblewski@xxxxxxxxx> wrote:
On 05/19/2014 02:44 PM, Jan Beulich wrote:
I used plural for a reason - I'm afraid you would need to start out with
taking them all, and then possibly determine which ones to drop as
being unrelated to the issue at hand.
Looks like a partial backport of your commit

commit aa9114edd97b292cd89b3616e3f2089471fd2201
Author: Jan Beulich <jbeulich@xxxxxxxx>
Date:   Thu Apr 10 16:01:41 2014 +0200

      x86/EPT: force re-evaluation of memory type as necessary

is all that's necessary. Attaching it versus 4.3.2. I only left the
memory_type_change calls in MTRR related areas, since only this is
problematic for the particular issue. This is probably good enough for
us, thanks for the pointers! Do you think this one is a relatively safe
for the stable branches?
I'm rather reluctant to put in any half-baked stuff like this - in going
through the set of changes you certainly noticed that there are
quite a few more fixes, that all deal with similar problems. So the
partial (and amended) backport you provided is really of the sort
"my problem is fixed, let's ignore everything else"...
Yup. Backporting the smallest viable fix seems ok solution for us, so I did it and we'll test it a bit, but I fully agree it's likely not the best thing for upstream project especially since that code seems very fresh. Given there doesn't seem to be much noise about recent (and very obvious) vm performance regressions with pci passthrough and large amounts of memory, it might be quite an unpopular use case to justify the risky backport.

Independently of this - Tim, do you think this EPT misconfig stuff can
already be considered mature enough for backporting? It's been in a
little over a month, and considering that we're using formally
undefined behavior here, I'd rather view this as not yet a backport


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