From: Toshi Kani <email@example.com> To: "Luis R. Rodriguez" <firstname.lastname@example.org> Cc: "Prarit Bhargava" <email@example.com>, "Stuart Hayes" <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, "H. Peter Anvin" <email@example.com>, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, "Toshi Kani" <email@example.com>, "Jan Beulich" <JBeulich@suse.com>, "Juergen Gross" <firstname.lastname@example.org>, "Roger Pau Monné" <email@example.com>, firstname.lastname@example.org Subject: Re: Fwd: [PATCH] x86: Use larger chunks in mtrr_cleanup Date: Fri, 04 Sep 2015 08:56:21 -0600 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <20150904014040.GA8051@wotan.suse.de> On Fri, 2015-09-04 at 03:40 +0200, Luis R. Rodriguez wrote: > On Thu, Sep 03, 2015 at 06:48:46PM -0600, Toshi Kani wrote: > > On Fri, 2015-09-04 at 01:54 +0200, Luis R. Rodriguez wrote: > > > On Thu, Sep 03, 2015 at 05:21:14PM -0600, Toshi Kani wrote: > > > > On Fri, 2015-09-04 at 00:45 +0200, Luis R. Rodriguez wrote: > > > > > On Thu, Sep 03, 2015 at 04:25:31PM -0600, Toshi Kani wrote: > > : > > > > > > On Xen, > > > > > > > > > > When Xen is used a platform firmware may still set up MTRR, even if > > > > > the hypervisor doesn't set up MTRR right ? So same issue and > > > > > question here. > > > > > > > > Right, I meant to say Xen guests. > > > > > > Ah but its import complicated than that. > > > > > > > In case of the Xen hypervisor, > > > > mtrr_type_lookup() returns a valid type as it runs on a platform. > > > > > > I am not sure if this happens today, I know MTRR is simply disabled by > > > the Xen Hypervisor on the CPU explicitly, it disable it so guests > > > reading the MTRR capabilities sees it as disabled when queried. > > > > Oh, I would not let the hypervisor to disable MTRRs... > > Commit 586ab6a055376ec3f3e1e8 ("x86/pvh: disable MTRR feature on cpuid for > Dom0") by Roger Pau Monné disables MTRR for PVH dom0, so that cpuid returns > that MTRR is disabled to guests. Oh, I see. It just clears the capability bit so that the kernel thinks MTRRs are disabled. That makes sense. > Then later on Linux as of commit 47591df50512 ("xen: Support Xen pv-domains > using PAT") added by Juergen as of v3.19 Linux guests can end up booting > without MTRR but with PAT now enabled. Nice! > > > Then since the Xen Linux guests cannot speak MTRR through the hypervisor > > > (for instance Xen guests cannot ask Xen hypervisor to mtrr_type_lookup() > > > for it) if PCI passthrough is used it could mean a guest might set up / > > > use incorrect info as well. > > > > > > If I undestand this correctly then I think we're in a pickle with Xen > > > unless we add hypervisor support and hypercall support for > > > mtrr_type_lookup(). > > > > I was under assumption that MTRRs are emulated and disabled on guests. > > Some "special" flavor Linux guests (with non-upstream code) have guest > MTRR hypercall support, for vanilla Xen and Linux they just never get MTRR > support. After Juergen's Linux changes though Xen guests can now get > shiny PAT support. Since MTRR hypercall support is not upstream and MTRR is > ancient I decided instead of adding MTRR hypercall support upstream to go > with converting all drivers to PAT interfaces, with the assumption there > would be no issues. > > > Isn't guest physical address virtualized? > > It is, there is a xen iotlb and stuff but that should ensure dom0 gets > to get proper access to devices, and if you use PCI passthrough you want > the best experience as well. > > > I know other proprietary VMMs on IA64, but know nothing about Xen... So, > > please disregard my comments to Xen. :-) > > No worries, no one knows all the answers, we work together to remove > cob webs off of these odd corners no one cares about :) Thanks for all the info! That helps. -Toshi
next prev parent reply other threads:[~2015-09-04 14:58 UTC|newest] Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <55E477DE.firstname.lastname@example.org> 2015-08-31 16:05 ` Stuart Hayes 2015-09-03 2:45 ` Luis R. Rodriguez 2015-09-03 12:17 ` Prarit Bhargava 2015-09-03 17:59 ` Luis R. Rodriguez 2015-09-03 18:10 ` Prarit Bhargava 2015-09-03 18:40 ` Luis R. Rodriguez 2015-09-03 19:22 ` Toshi Kani 2015-09-03 19:51 ` Luis R. Rodriguez 2015-09-03 21:31 ` Toshi Kani 2015-09-03 22:07 ` Luis R. Rodriguez 2015-09-03 22:25 ` Toshi Kani 2015-09-03 22:45 ` Luis R. Rodriguez 2015-09-03 23:21 ` Toshi Kani 2015-09-03 23:54 ` Luis R. Rodriguez 2015-09-04 0:48 ` Toshi Kani 2015-09-04 1:40 ` Luis R. Rodriguez 2015-09-04 14:56 ` Toshi Kani [this message] 2015-09-04 6:51 ` Jan Beulich 2015-09-14 14:46 ` Stuart Hayes 2015-11-05 19:14 ` Yinghai Lu 2015-11-05 19:43 ` Luis R. Rodriguez 2016-03-16 20:20 ` Luis R. Rodriguez 2016-03-29 17:07 ` Luis R. Rodriguez
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