linux-kernel - Re: [PATCH v2 0/4] iommu/amd: Enable page-selective flushes

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Date:   Fri, 4 Jun 2021 12:57:35 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Jiajun Cao <caojiajun@...are.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/4] iommu/amd: Enable page-selective flushes

> On Jun 4, 2021, at 11:53 AM, Robin Murphy <robin.murphy@....com> wrote:
> 
> On 2021-06-04 18:10, Nadav Amit wrote:
>>> On Jun 4, 2021, at 8:38 AM, Joerg Roedel <joro@...tes.org> wrote:
>>> 
>>> Hi Nadav,
>>> 
>>> [Adding Robin]
>>> 
>>> 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
>>> 
>>> 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.
>>> 
>>>>  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
>>> 
>>> And I think there have been objections from Robin Murphy on Patch 3,
>>> have those been worked out?
>> I am still waiting for Robin’s feedback on my proposed changes. If he does not respond soon, I will drop this patch for now.
> 
> 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"...
> 
> 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’s 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’t 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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