On 8/15/22 13:51, Christian König wrote: > Am 15.08.22 um 12:47 schrieb Dmitry Osipenko: >> On 8/15/22 13:18, Dmitry Osipenko wrote: >>> On 8/15/22 13:14, Christian König wrote: >>>> Am 15.08.22 um 12:11 schrieb Christian König: >>>>> Am 15.08.22 um 12:09 schrieb Dmitry Osipenko: >>>>>> On 8/15/22 13:05, Christian König wrote: >>>>>>> Am 15.08.22 um 11:54 schrieb Dmitry Osipenko: >>>>>>>> Higher order pages allocated using alloc_pages() aren't >>>>>>>> refcounted and >>>>>>>> they >>>>>>>> need to be refcounted, otherwise it's impossible to map them by >>>>>>>> KVM. This >>>>>>>> patch sets the refcount of the tail pages and fixes the KVM memory >>>>>>>> mapping >>>>>>>> faults. >>>>>>>> >>>>>>>> Without this change guest virgl driver can't map host buffers into >>>>>>>> guest >>>>>>>> and can't provide OpenGL 4.5 profile support to the guest. The host >>>>>>>> mappings are also needed for enabling the Venus driver using >>>>>>>> host GPU >>>>>>>> drivers that are utilizing TTM. >>>>>>>> >>>>>>>> Based on a patch proposed by Trigger Huang. >>>>>>> Well I can't count how often I have repeated this: This is an >>>>>>> absolutely >>>>>>> clear NAK! >>>>>>> >>>>>>> TTM pages are not reference counted in the first place and >>>>>>> because of >>>>>>> this giving them to virgl is illegal. >>>>>> A? The first page is refcounted when allocated, the tail pages are >>>>>> not. >>>>> No they aren't. The first page is just by coincident initialized with >>>>> a refcount of 1. This refcount is completely ignored and not used >>>>> at all. >>>>> >>>>> Incrementing the reference count and by this mapping the page into >>>>> some other address space is illegal and corrupts the internal state >>>>> tracking of TTM. >>>> See this comment in the source code as well: >>>> >>>> /* Don't set the __GFP_COMP flag for higher order allocations. >>>> * Mapping pages directly into an userspace process and >>>> calling >>>> * put_page() on a TTM allocated page is illegal. >>>> */ >>>> >>>> I have absolutely no idea how somebody had the idea he could do this. >>> I saw this comment, but it doesn't make sense because it doesn't explain >>> why it's illegal. Hence it looks like a bogus comment since the >>> refcouting certainly works, at least to a some degree because I haven't >>> noticed any problems in practice, maybe by luck :) >>> >>> I'll try to dig out the older discussions, thank you for the quick >>> reply! >> Are you sure it was really discussed in public previously? All I can >> find is yours two answers to a similar patches where you're saying that >> this it's a wrong solution without in-depth explanation and further >> discussions. > > Yeah, that's my problem as well I can't find that of hand. > > But yes it certainly was discussed in public. If it was only CC'd to dri-devel, then could be that emails didn't pass the spam moderation :/ >> Maybe it was discussed privately? In this case I will be happy to get >> more info from you about the root of the problem so I could start to >> look at how to fix it properly. It's not apparent where the problem is >> to a TTM newbie like me. >> > > Well this is completely unfixable. See the whole purpose of TTM is to > allow tracing where what is mapped of a buffer object. > > If you circumvent that and increase the page reference yourself than > that whole functionality can't work correctly any more. Are you suggesting that the problem is that TTM doesn't see the KVM page faults/mappings? -- Best regards, Dmitry
