Re: [PATCH] iommu/vt-d: Remove unnecassary qi clflushes

[Nadav Amit <nadav.amit-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Nadav Amit <nadav.amit@gmail.com> wrote:

> Joerg Roedel <joro@8bytes.org> wrote:
> 
>> On Fri, Jun 24, 2016 at 06:13:14AM -0700, Nadav Amit wrote:
>>> According to the manual: "Hardware access to ...  invalidation queue ...
>>> are always coherent."
>>> 
>>> Remove unnecassary clflushes accordingly.
>> 
>> It is one thing what the spec says and another how hardware really
>> behaves. Have you tested this on (potentially really old) VT-d machines
>> to make sure the spec is _always_ right here?
> 
> No I didn’t as the commit message says. I would be happy for someones’
> tested-by.
> 
> Having said that - FreeBSD does not do these (unnecessary)
> invalidations [1], and their code comment clearly says it. Since this code
> is not new, I would assume FreeBSD would crash by now if the code was
> buggy.
> 
> Although such hardware is old, there are some hypervisors that do not set
> the ecap.coherency of emulated IOMMUs. Yes, it is unwise, but there is no
> reason to further punish these hypervisors.

Ping? To clarify - this behavior applies to the guest of VMware and KVM
(which uses QEMU for IOMMU emulation).

Regardless, please respond to the other patch I sent as well.

Thanks,
Nadav

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