linux-kernel - Re: [PATCH v2 0/2] iommu/amd: Don't use atomic64_t for domain->pt

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Message-ID: <20200630095721.GI28824@8bytes.org>
Date:   Tue, 30 Jun 2020 11:57:21 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Qian Cai <cai@....pw>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH v2 0/2] iommu/amd: Don't use atomic64_t for
 domain->pt_root

On Fri, Jun 26, 2020 at 08:30:21AM -0400, Qian Cai wrote:
> BTW, from the previous discussion, Linus mentioned,
>  
> “
> The thing is, the 64-bit atomic reads/writes are very expensive on
> 32-bit x86. If it was just a native pointer, it would be much cheaper
> than an "atomic64_t".
> “
> 
> However, here we have AMD_IOMMU depend on x86_64, so I am wondering if
> it makes any sense to run this code on 32-bit x86 at all?

No, it doesn't, the driver is not supported on 32bit and probably never
will. I skip this patch and only apply the first one, as it is an
improvement in itself.

Regards,

	Joerg