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From: Toshi Kani <toshi.kani@hpe.com>
To: "Luis R. Rodriguez" <mcgrof@suse.com>
Cc: "Prarit Bhargava" <prarit@redhat.com>,
	"Stuart Hayes" <stuart.w.hayes@gmail.com>,
	tglx@linutronix.de, mingo@redhat.com,
	"H. Peter Anvin" <hpa@zytor.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	mcgrof@do-not-panic.com, "Toshi Kani" <toshi.kani@hp.com>,
	"Jan Beulich" <JBeulich@suse.com>,
	"Juergen Gross" <jgross@suse.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>,
	xen-devel@lists.xensource.com
Subject: Re: Fwd: [PATCH] x86: Use larger chunks in mtrr_cleanup
Date: Fri, 04 Sep 2015 08:56:21 -0600	[thread overview]
Message-ID: <1441378581.21638.9.camel@hpe.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

  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.2060106@gmail.com>
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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