[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] x86/CPUID: suppress IOMMU related hypervisor leaf data
On 28.12.2020 11:54, Roger Pau Monné wrote: > 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. I'm afraid I don't understand your concern and/or description of the scenario. Prior to the change, the bit was set unconditionally. To me, _that_ was making the bit useless - no point in checking something which is always set anyway (leaving aside old Xen versions). I also don't follow how presence / absence of a passed through device mattering (or not) would have changed with this patch. Jan
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |