linux-kernel - Re: [PATCH v6] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

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Message-ID: <69A79870-3822-44AA-8A5D-DC329828CF64@amd.com>
Date:   Tue, 24 Nov 2020 09:43:37 +0000
From:   "Kalra, Ashish" <Ashish.Kalra@....com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        "hch@....de" <hch@....de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "dave.hansen@...ux-intel.com" <dave.hansen@...ux-intel.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Singh, Brijesh" <brijesh.singh@....com>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        "Grimm, Jon" <Jon.Grimm@....com>,
        "rientjes@...gle.com" <rientjes@...gle.com>
Subject: Re: [PATCH v6] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.



> On Nov 24, 2020, at 3:38 AM, Borislav Petkov <bp@...en8.de> wrote:
> 
> On Tue, Nov 24, 2020 at 09:25:06AM +0000, Kalra, Ashish wrote:
>> But what will be the criteria to figure out this percentage?
>> 
>> As I mentioned earlier, this can be made as complicated as possible by
>> adding all kind of heuristics but without any predictable performance
>> gain.
>> 
>> Or it can be kept simple by using a static percentage value.
> 
> Yes, static percentage number based on the guest memory. X% of the guest
> memory is used for SWIOTLB.
> 
> Since you use sev_active(), it means the size computation is done in the
> guest so that SWIOTLB size is per-guest. Yes?

Yes

> 
> If so, you can simply take, say, 5% of the guest memory's size and use
> that for SWIOTLB buffers. Or 6 or X or whatever.
> 
> Makes sense?

Sure it does.

Thanks,
Ashish

> 
> -- 
> Regards/Gruss,
>    Boris.
> 
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