On Tue, Sep 06, 2022, Daniel Vetter wrote: > On Tue, Sep 06, 2022 at 10:01:47PM +0200, Daniel Vetter wrote: > > On Mon, Aug 15, 2022 at 12:05:19PM +0200, 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. > > > > > > Please immediately stop this completely broken approach. We have discussed > > > this multiple times now. > > > > Yeah we need to get this stuff closed for real by tagging them all with > > VM_IO or VM_PFNMAP asap. > > For a bit more context: Anything mapping a bo should be VM_SPECIAL. And I > think we should add the checks to the gem and dma-buf mmap functions to > validate for that, and fix all the fallout. > > Otherwise this dragon keeps resurrecting ... > > VM_SPECIAL _will_ block get_user_pages, which will block everyone from > even trying to refcount this stuff. FWIW, IIUC that won't change the KVM story. KVM acquires the PFN for these pages via follow_pte(), not by gup(). Details are in a different strand of this thread[*]. If TTM pages aren't tied into mmu_notifiers, then I believe the only solution is to not allow them to be mapped into user page tables. If they are tied into mmu_notifiers, then this is fully a KVM limitation that we are (slowly) resolving. [*] https://lore.kernel.org/all/Y77sQZI0IfFVx7Jo@xxxxxxxxxx
