question on share ept page table with vtd

question on share ept page table with vtd

[Zhang, Yang Z]

Hi all

I have a question with the page table share between EPT and VTD which is enabled by default. Currently, we will enable log_dirty mode to track vram. And when first enable the log dirty mode, it will set all guest ram to readonly in EPT entry. The question is that if a page is current used as DMA write buffer, isn't it a problem?

best regards
yang

Re: question on share ept page table with vtd

[Andrew Cooper]

On 01/02/2014 00:29, Zhang, Yang Z wrote:
> Hi all
>
> I have a question with the page table share between EPT and VTD which is enabled by default. Currently, we will enable log_dirty mode to track vram. And when first enable the log dirty mode, it will set all guest ram to readonly in EPT entry. The question is that if a page is current used as DMA write buffer, isn't it a problem?
>
> best regards
> yang

Looking at the code, it would indeed appear to be a problem.

Is has presumably gone unnoticed until now as all writes into the vram
would have been from software, and hit the log_dirty logic, rather than
DMA writes hitting the IOMMU.

~Andrew

Re: question on share ept page table with vtd

[Zhang, Yang Z]

Andrew Cooper wrote on 2014-02-01:
> On 01/02/2014 00:29, Zhang, Yang Z wrote:
>> Hi all
>> 
>> I have a question with the page table share between EPT and VTD
>> which is
> enabled by default. Currently, we will enable log_dirty mode to track
> vram. And when first enable the log dirty mode, it will set all guest
> ram to readonly in EPT entry. The question is that if a page is
> current used as DMA write buffer, isn't it a problem?
>> 
>> best regards
>> yang
> 
> Looking at the code, it would indeed appear to be a problem.
> 
> Is has presumably gone unnoticed until now as all writes into the vram
> would have been from software, and hit the log_dirty logic, rather
> than DMA writes hitting the IOMMU.
> 

Yes, and I do saw the issue that caused by this that SRIOV fails issue with Xen 4.4+qemu-xen-dir. 
http://www.gossamer-threads.com/lists/xen/devel/315663

There are two places that will call paging_log_dirty_enable() to enable log dirty mode: one is in vram tracking and on is in domain saving. Since VT-d is not recommended to use with saving/restore, so for latter case, it is OK. But for the vram tracking, I don't think we need to set all guest ram to readonly when first enable log dirty mode. Only set the memory range that used by vram is enough.
Furthermore, I think sharing EPT page table with VT-d is not a good idea since hypervisor can mark the ram as readonly in any time and in any usage mode in furture. If sharing page table is used, this will becomes impossible. 

> ~Andrew


Best regards,
Yang