On 1/18/19 1:43 PM, Julien Grall wrote: > (+ Stefano) > > Hi, > > Sorry for jumping late in the conversation. > > On 18/01/2019 09:40, Oleksandr Andrushchenko wrote: >> On 1/17/19 11:18 AM, Christoph Hellwig wrote: >>> On Wed, Jan 16, 2019 at 06:43:29AM +0000, Oleksandr Andrushchenko >>> wrote: >>>>> This whole issue keeps getting more and more confusing. >>>> Well, I don't really do DMA here, but instead the buffers in >>>> question are shared with other Xen domain, so effectively it >>>> could be thought of some sort of DMA here, where the "device" is >>>> that remote domain. If the buffers are not flushed then the >>>> remote part sees some inconsistency which in my case results >>>> in artifacts on screen while displaying the buffers. >>>> When buffers are allocated via DMA API then there are no artifacts; >>>> if buffers are allocated with shmem + DMA mapping then there are no >>>> artifacts as well. >>>> The only offending use-case is when I use shmem backed buffers, >>>> but do not flush them >>> The right answer would be to implement cache maintainance hooks for >>> this case in the Xen arch code. These would basically look the same >>> as the low-level cache maintainance used by the DMA ops, but without >>> going through the DMA mapping layer, in fact they should probably >>> reuse the same low-level assembly routines. >>> >>> I don't think this is the first usage of such Xen buffer sharing, so >>> what do the other users do? >> I'll have to get even deeper into it. Initially I >> looked at the code, but didn't find anything useful. >> Or maybe I have just overlooked obvious things there > From Xen on Arm ABI: > > "All memory which is shared with other entities in the system > (including the hypervisor and other guests) must reside in memory > which is mapped as Normal Inner Write-Back Outer Write-Back > Inner-Shareable. > This applies to: > - hypercall arguments passed via a pointer to guest memory. > - memory shared via the grant table mechanism (including PV I/O > rings etc). > - memory shared with the hypervisor (struct shared_info, struct > vcpu_info, the grant table, etc). > " > > So you should not need any cache maintenance here. Can you provide > more details on the memory attribute you use for memory shared in both > the backend and frontend? > It takes quite some time to collect this (because many components are involved in the use-case), but for now the pages in the guest domain are: !PTE_RDONLY + PTE_PXN + PTE_SHARED + PTE_AF + PTE_UXN + PTE_ATTRINDX(MT_NORMAL)
> Cheers, > >> >> Thank you, >> Oleksandr >> _______________________________________________ >> Xen-devel mailing list >> xen-de...@lists.xenproject.org >> https://lists.xenproject.org/mailman/listinfo/xen-devel >> >
