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Peter Anvin" , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Shuah Khan , Shuah Khan , Vishal Annapurve , Andrew Morton , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Youngjun Park , Qi Zheng , Shakeel Butt , Kiryl Shutsemau , Jason Gunthorpe , Vlastimil Babka , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev Content-Type: text/plain; charset="us-ascii" On Thu, May 21, 2026, Ackerley Tng wrote: > Sean Christopherson writes: > > > On Thu, May 21, 2026, Fuad Tabba wrote: > >> Hi, > >> > >> On Thu, 7 May 2026 at 21:22, Ackerley Tng via B4 Relay > > diff --git include/linux/kvm_host.h include/linux/kvm_host.h > > index 61a3430957f2..b83cda2870ba 100644 > > --- include/linux/kvm_host.h > > +++ include/linux/kvm_host.h > > @@ -2596,7 +2596,8 @@ int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_ord > > typedef int (*kvm_gmem_populate_cb)(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, > > struct page *page, void *opaque); > > > > -long kvm_gmem_populate(struct kvm *kvm, gfn_t gfn, void __user *src, long npages, > > +long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, > > + long npages, bool writable, > > What do you think of need_writable_src instead of just writable for the > variable name? How about "may_write_src" or "may_writeback_src"? > > kvm_gmem_populate_cb post_populate, void *opaque); > > #endif > > > > diff --git virt/kvm/guest_memfd.c virt/kvm/guest_memfd.c > > index a35a55571a2d..6553d4e032ce 100644 > > --- virt/kvm/guest_memfd.c > > +++ virt/kvm/guest_memfd.c > > @@ -858,7 +858,8 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot, > > return ret; > > } > > > > -long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long npages, > > +long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, > > + long npages, bool writable, > > kvm_gmem_populate_cb post_populate, void *opaque) > > { > > struct kvm_memory_slot *slot; > > @@ -892,8 +893,9 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long > > > > if (src) { > > unsigned long uaddr = (unsigned long)src + i * PAGE_SIZE; > > + unsigned int flags = writable ? FOLL_WRITE : 0; > > How about using FOLL_WRITE | FOLL_NOFAULT so if it weren't writable to > start with, don't CoW, just error out? Eh, I don't see any value in value in erroring out if userspace is doing something unusual. If breaking CoW was actually problematic somehow, then sure. But AFAICT it's overall harmless. > Like you said above the CPUID page provided as src_page would have been > written to before, so it should have been mapped as writable.