linux-kernel - Re: [PATCH v7] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

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Message-ID: <60FF61F4-9A7D-4467-A148-2956903C74AA@amd.com>
Date:   Mon, 7 Dec 2020 22:20:20 +0000
From:   "Kalra, Ashish" <Ashish.Kalra@....com>
To:     Borislav Petkov <bp@...en8.de>
CC:     "konrad.wilk@...cle.com" <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 v7] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.



> On Dec 7, 2020, at 4:14 PM, Borislav Petkov <bp@...en8.de> wrote:
> 
> On Mon, Dec 07, 2020 at 10:06:24PM +0000, Ashish Kalra wrote:
>> This is related to the earlier static adjustment of the SWIOTLB buffers
>> as per guest memory size and Konrad's feedback on the same, as copied
>> below : 
>> 
>>>> That is eating 128MB for 1GB, aka 12% of the guest memory allocated statically for this.
>>>> 
>>>> And for guests that are 2GB, that is 12% until it gets to 3GB when 
>>>> it is 8% and then 6% at 4GB.
>>>> 
>>>> I would prefer this to be based on your memory count, that is 6% of 
>>>> total memory.
> 
> So no rule of thumb and no measurements? Just a magic number 6.

It is more of an approximation of the earlier static adjustment which was 128M for <1G guests, 256M for 1G-4G guests and 512M for >4G guests.

Thanks,
Ashish