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Re: [PATCH] x86/CPUID: suppress IOMMU related hypervisor leaf data
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 28 Dec 2020 11:54:00 +0100
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On Mon, Nov 09, 2020 at 11:54:09AM +0100, Jan Beulich wrote:
> Now that the IOMMU for guests can't be enabled "on demand" anymore,
> there's also no reason to expose the related CPUID bit "just in case".
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
I'm not sure this is helpful from a guest PoV.
How does the guest know whether it has pass through devices, and thus
whether it needs to check if this flag is present or not in order to
safely pass foreign mapped pages (or grants) to the underlying devices?
Ie: prior to this change I would just check whether the flag is
present in CPUID to know whether FreeBSD needs to use a bounce buffer
in blkback and netback when running as a domU. If this is now
conditionally set only when the IOMMU is enabled for the guest I
also need to figure a way to know whether the domU has any passed
through device or not, which doesn't seem trivial.
Roger.
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