linux-kernel - Re: [Xen-devel] [PATCH v2 1/3] xen/pvh: enable and set default MTRR type

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Message-ID: <20180510115206.nbkthffhksy3l4g5@citrix.com>
Date:   Thu, 10 May 2018 12:52:06 +0100
From:   Wei Liu <wei.liu2@...rix.com>
To:     Roger Pau Monné <roger.pau@...rix.com>
CC:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        <linux-kernel@...r.kernel.org>, Wei Liu <wei.liu2@...rix.com>
Subject: Re: [Xen-devel] [PATCH v2 1/3] xen/pvh: enable and set default MTRR
 type

On Wed, May 09, 2018 at 04:11:39PM +0100, Roger Pau Monné wrote:
> On Wed, May 09, 2018 at 12:30:16PM +0100, Roger Pau Monné wrote:
> > On Wed, May 09, 2018 at 11:56:40AM +0100, Andrew Cooper wrote:
> > > On 09/05/18 11:21, Roger Pau Monne wrote:
> > > I'm not sure that setting the default MTRR type is going to be a
> > > clever idea in hindsight when we come to doing PCI Passthrough support.
> > 
> > Setting the default type to WB is also set by hvmloader, it's just
> > that hvmloader also sets some of the fixed and variable ranges to UC
> > in order to cover the iomem areas.
> > 
> > The expectations when doing pci-passthrough is that the guest will
> > always use paging and PAT in order to set the appropriate cache
> > attributes, or else the guest itself will have to program the UC MTRR
> > ranges, I admit that's not very nice however.
> > 
> > What about enabling the default MTRR type and setting it to WB in the
> > toolstack for PVH? IMO doing it Xen itself would be wrong.
> 
> I have the following patch to set the default MTRR type, but I think
> if we go down this road then we will also have to set UC MTRRs for
> MMIO areas, which again seems fine to me.
> 

Can you please document the default type(s) to pvh.markdown once
the issue is resolved?

Wei.