Re: [Xen-devel] [PATCH] libxc: Expose the pdpe1gb cpuid flag to guest

[Tim Deegan <tim@xxxxxxx>]

At 16:24 +0000 on 17 Nov (1416237888), Jan Beulich wrote:
> >>> On 17.11.14 at 16:39, <Ian.Jackson@xxxxxxxxxxxxx> wrote:
> > Liang Li writes ("[PATCH] libxc: Expose the pdpe1gb cpuid flag to guest"):
> >> If hardware support the pdpe1gb flag, expose it to guest by default.
> >> Users don't have to use a 'cpuid= ' option in config file to turn
> >> it on.
> > 
> > I don't understand what this flag does.  I guess from the name it
> > turns on 1G pages.  I guess these are supported ?
> > 
> > I would like to see comment from an x86 hypervisor maintainer.
> 
> Yes, we support 1Gb pages. The purpose of the patch is to not
> unconditionally hide the flag from guests. (I had commented on
> v1, but sadly this one wasn't tagged as v2, nor was I included on
> the Cc list, hence I didn't spot the new version.)
> 
> The one thing I'm not certain about is shadow mode: Only 2Mb
> pages may be supported there. Tim?

Indeed, only 2MiB pages are supported in shadow mode.  See, e.g.
guest_supports_1G_superpages()->hvm_pse1gb_supported()->paging_mode_hap()

Tim.

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