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From:   Nadav Amit <nadav.amit@gmail.com>
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Subject: Re: [PATCH v2 0/4] iommu/amd: Enable page-selective flushes
Date:   Fri, 4 Jun 2021 12:57:35 -0700
In-Reply-To: <2ea809ef-beb5-a2c9-0739-cb236cab196b@arm.com>
Cc:     Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Jiajun Cao <caojiajun@vmware.com>,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org
To:     Robin Murphy <robin.murphy@arm.com>
References: <20210524224159.32807-1-namit@vmware.com>
 <YLpI7tKtsf4l5MlN@8bytes.org>
 <05098022-1ED6-44BE-931D-D16C2D0B2D09@gmail.com>
 <2ea809ef-beb5-a2c9-0739-cb236cab196b@arm.com>
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> On Jun 4, 2021, at 11:53 AM, Robin Murphy <robin.murphy@arm.com> =
wrote:
>=20
> On 2021-06-04 18:10, Nadav Amit wrote:
>>> On Jun 4, 2021, at 8:38 AM, Joerg Roedel <joro@8bytes.org> wrote:
>>>=20
>>> Hi Nadav,
>>>=20
>>> [Adding Robin]
>>>=20
>>> On Mon, May 24, 2021 at 03:41:55PM -0700, Nadav Amit wrote:
>>>> Nadav Amit (4):
>>>>  iommu/amd: Fix wrong parentheses on page-specific invalidations
>>>=20
>>> This patch is already upstream in v5.13-rc4. Please rebase to that
>>> version.
>> I guess it would be rc5 by the time I send it.
>>>=20
>>>>  iommu/amd: Selective flush on unmap
>>>>  iommu/amd: Do not sync on page size changes
>>>>  iommu/amd: Do not use flush-queue when NpCache is on
>>>=20
>>> And I think there have been objections from Robin Murphy on Patch 3,
>>> have those been worked out?
>> I am still waiting for Robin=E2=80=99s feedback on my proposed =
changes. If he does not respond soon, I will drop this patch for now.
>=20
> Apologies, it feels like I've spent most of this week fighting fires,
> and a great deal of email got skimmed and mentally filed under =
"nothing
> so wrong that I need to respond immediately"...
>=20
> FWIW I would have written the simpler patch below, but beyond that I
> think it might start descending into bikeshedding - if you still =
prefer
> your more comprehensive refactoring, or something in between, then =
don't
> let my personal preference in style/complexity trade-offs stand in the
> way of getting a useful functional change into the AMD driver. =
Whichever
> way, though, I *am* now sold on the idea of having some kerneldoc to
> clarify these things.

Thanks, I appreciate your feedback.

I will add kerneldoc as you indicated.

I see you took some parts of the patch I did for MediaTek, but I think =
this is not good enough for AMD, since AMD behavior should be different =
than MediaTek - they have different needs:

MediaTek wants as few IOTLB flushes as possible, even if it results in =
flushing of many irrelevant (unmodified) entries between start and end. =
That=E2=80=99s the reason it can just use =
iommu_iotlb_gather_update_range().

In contrast, for AMD we do not want to flush too many irrelevant =
entries, specifically if the the IOMMU is virtualized. When an IOTLB =
flush is initiated by the VM, the hypervisor needs to scan the IOMMU =
page-tables for changes and synchronize it with the physical IOMMU. You =
don=E2=80=99t want this range to be too big, and that is the reason I =
needed iommu_iotlb_gather_is_disjoint().

I will add documentation, since clearly this information was not =
conveyed well enough.

Thanks again,
Nadav

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