linux-kernel - Re: [PATCH] iommu/amd: page-specific invalidations for more than one page

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Message-ID: <YG2C42UdIEsWex2L@8bytes.org>
Date:   Wed, 7 Apr 2021 12:01:07 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Will Deacon <will@...nel.org>, Nadav Amit <namit@...are.com>,
        Jiajun Cao <caojiajun@...are.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/amd: page-specific invalidations for more than one
 page

On Tue, Mar 23, 2021 at 02:06:19PM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@...are.com>
> 
> Currently, IOMMU invalidations and device-IOTLB invalidations using
> AMD IOMMU fall back to full address-space invalidation if more than a
> single page need to be flushed.
> 
> Full flushes are especially inefficient when the IOMMU is virtualized by
> a hypervisor, since it requires the hypervisor to synchronize the entire
> address-space.
> 
> AMD IOMMUs allow to provide a mask to perform page-specific
> invalidations for multiple pages that match the address. The mask is
> encoded as part of the address, and the first zero bit in the address
> (in bits [51:12]) indicates the mask size.
> 
> Use this hardware feature to perform selective IOMMU and IOTLB flushes.
> Combine the logic between both for better code reuse.
> 
> The IOMMU invalidations passed a smoke-test. The device IOTLB
> invalidations are untested.

Have you thoroughly tested this on real hardware? I had a patch-set
doing the same many years ago and it lead to data corruption under load.
Back then it could have been a bug in my code of course, but it made me
cautious about using targeted invalidations.

Regards,

	Joerg