linux-kernel - Re: [RFC V1 1/5] swiotlb: Support allocating DMA memory from SWIOTLB

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Date: Wed, 14 Feb 2024 16:49:43 +0200
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Vishal Annapurve <vannapurve@...gle.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, pbonzini@...hat.com, 
	rientjes@...gle.com, bgardon@...gle.com, seanjc@...gle.com, erdemaktas@...gle.com, 
	ackerleytng@...gle.com, jxgao@...gle.com, sagis@...gle.com, oupton@...gle.com, 
	peterx@...hat.com, vkuznets@...hat.com, dmatlack@...gle.com, pgonda@...gle.com, 
	michael.roth@....com, thomas.lendacky@....com, dave.hansen@...ux.intel.com, 
	linux-coco@...ts.linux.dev, chao.p.peng@...ux.intel.com, isaku.yamahata@...il.com, 
	andrew.jones@...ux.dev, corbet@....net, hch@....de, m.szyprowski@...sung.com, bp@...e.de, 
	rostedt@...dmis.org, iommu@...ts.linux.dev
Subject: Re: [RFC V1 1/5] swiotlb: Support allocating DMA memory from SWIOTLB

On Fri, Jan 12, 2024 at 05:52:47AM +0000, Vishal Annapurve wrote:
> Modify SWIOTLB framework to allocate DMA memory always from SWIOTLB.
> 
> CVMs use SWIOTLB buffers for bouncing memory when using dma_map_* APIs
> to setup memory for IO operations. SWIOTLB buffers are marked as shared
> once during early boot.
> 
> Buffers allocated using dma_alloc_* APIs are allocated from kernel memory
> and then converted to shared during each API invocation. This patch ensures
> that such buffers are also allocated from already shared SWIOTLB
> regions. This allows enforcing alignment requirements on regions marked
> as shared.

But does it work in practice? 

Some devices (like GPUs) require a lot of DMA memory. So with this approach
we would need to have huge SWIOTLB buffer that is unused in most VMs.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov