linux-kernel - Re: [RFC PATCH V3 08/11] swiotlb: Add bounce buffer remap address setting function

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Message-ID: <9c05f7fd-6460-5d4a-aa83-08626839d18e@gmail.com>
Date:   Thu, 10 Jun 2021 22:25:10 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de,
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        hannes@...xchg.org, cai@....pw, krish.sadhukhan@...cle.com,
        saravanand@...com, Tianyu.Lan@...rosoft.com,
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Subject: Re: [RFC PATCH V3 08/11] swiotlb: Add bounce buffer remap address
 setting function



On 6/7/2021 10:56 PM, Tianyu Lan wrote:
> 
> On 6/7/2021 2:43 PM, Christoph Hellwig wrote:
>> On Sun, May 30, 2021 at 11:06:25AM -0400, Tianyu Lan wrote:
>>> From: Tianyu Lan <Tianyu.Lan@...rosoft.com>
>>>
>>> For Hyper-V isolation VM with AMD SEV SNP, the bounce buffer(shared 
>>> memory)
>>> needs to be accessed via extra address space(e.g address above bit39).
>>> Hyper-V code may remap extra address space outside of swiotlb. swiotlb_
>>> bounce() needs to use remap virtual address to copy data from/to bounce
>>> buffer. Add new interface swiotlb_set_bounce_remap() to do that.
>>
>> Why can't you use the bus_dma_region ranges to remap to your preferred
>> address?
>>
> 
> Thanks for your suggestion.
> 
> 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. During 
> initializing swiotlb bounce buffer pool, only address bellow shared gpa 
> boundary can be accepted by swiotlb API because it is treated as system 
> memory and managed by memory management. This is why Hyper-V swiotlb 
> bounce buffer pool needs to be allocated in Hyper-V code and map
> associated physical address in extra address space. The patch target is
> to add the new interface to set start virtual address of bounce buffer
> pool and let swiotlb boucne buffer copy function to use right virtual 
> address for extra address space.
> 
> bus_dma_region is to translate cpu physical address to dma address.
> It can't modify the virtual address of bounce buffer pool and let
> swiotlb code to copy data with right address. If some thing missed,
> please correct me.
> 

Hi Christoph:
	Sorry to bother you. Could you have a look at my previous reply?
I try figuring out the right way.

Thanks.