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Subject: Re: [RFC PATCH V3 08/11] swiotlb: Add bounce buffer remap address
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To:     Christoph Hellwig <hch@lst.de>
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References: <20210530150628.2063957-1-ltykernel@gmail.com>
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 <20210607064312.GB24478@lst.de>
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 <20210614071223.GA30171@lst.de>
From:   Tianyu Lan <ltykernel@gmail.com>
Message-ID: <3e64e59b-7440-69a5-75c5-43225f3d6c0a@gmail.com>
Date:   Mon, 14 Jun 2021 21:37:01 +0800
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On 6/14/2021 3:12 PM, Christoph Hellwig wrote:
> On Mon, Jun 07, 2021 at 10:56:47PM +0800, Tianyu Lan wrote:
>> These addresses in extra address space works as system memory mirror. The
>> shared memory with host in Isolation VM needs to be accessed via extra
>> address space which is above shared gpa boundary.
> 
> Why?
> 

The shared_gpa_boundary in the AMD SEV SNP spec is called virtual top of
memory(vTOM). Memory addresses below vTOM are automatically treated as
private while memory above vTOM is treated as shared. Using vTOM to
separate memory in this way avoids the need to augment the standard x86
page tables with C-bit markings, simplifying guest OS software.