Thanks for taking the time and sorry for the delay. > On 6. Apr 2017, at 16:22, Radim Krčmář <[email protected]> wrote: > > 2017-04-05 15:07+0200, Filippo Sironi: >> cmpxchg_gpte() calls get_user_pages_fast() to retrieve the number of >> pages and the respective struct pages for mapping in the kernel virtual >> address space. >> This doesn't work if get_user_pages_fast() is invoked with a userspace >> virtual address that's backed by PFNs outside of kernel reach (e.g., >> when limiting the kernel memory with mem= in the command line and using >> /dev/mem to map memory). >> >> If get_user_pages_fast() fails, look up the VMA that backs the userspace >> virtual address, compute the PFN and the physical address, and map it in >> the kernel virtual address space with memremap(). > > What is the reason for a configuration that voluntarily restricts access > to memory that it needs?
By using /dev/mem to provide VM memory, one can avoid the overhead of allocating struct page(s) for the whole memory, which is wasteful when using a server entirely for hosting VMs. >> Signed-off-by: Filippo Sironi <[email protected]> >> Cc: Anthony Liguori <[email protected]> >> Cc: [email protected] >> Cc: [email protected] >> --- >> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h >> @@ -147,15 +147,36 @@ static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, >> struct kvm_mmu *mmu, >> struct page *page; >> >> npages = get_user_pages_fast((unsigned long)ptep_user, 1, 1, &page); >> - /* Check if the user is doing something meaningless. */ >> - if (unlikely(npages != 1)) >> - return -EFAULT; >> - >> - table = kmap_atomic(page); >> - ret = CMPXCHG(&table[index], orig_pte, new_pte); >> - kunmap_atomic(table); >> - >> - kvm_release_page_dirty(page); >> + if (likely(npages == 1)) { >> + table = kmap_atomic(page); >> + ret = CMPXCHG(&table[index], orig_pte, new_pte); >> + kunmap_atomic(table); >> + >> + kvm_release_page_dirty(page); >> + } else { >> + struct vm_area_struct *vma; >> + unsigned long vaddr = (unsigned long)ptep_user & PAGE_MASK; >> + unsigned long pfn; >> + unsigned long paddr; >> + >> + down_read(¤t->mm->mmap_sem); >> + vma = find_vma_intersection(current->mm, vaddr, >> + vaddr + PAGE_SIZE); > > Hm, with the argument order like this, we check that > > vaddr < vma->vm_end && vaddr + PAGE_SIZE > vma->vm_start > > but shouldn't we actually check that the whole page is there, i.e. > > vaddr + PAGE_SIZE < vma->vm_end && vaddr > vma->vm_start > > ? > > Thanks. Hm, right now we check for the following: vaddr >= vma->vm_start && vaddr < vma->vm_end && vaddr + PAGE_SIZE > vma->vm_start given that vaddr is PAGE_SIZE aligned, we're guaranteed that vaddr + PAGE_SIZE <= vma->vm_end. This seems more complex than necessary. I believe that: vma = find_vma(current->mm, vaddr); should be enough. >> + if (!vma || !(vma->vm_flags & VM_PFNMAP)) { >> + up_read(¤t->mm->mmap_sem); >> + return -EFAULT; >> + } >> + pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff; >> + paddr = pfn << PAGE_SHIFT; >> + table = memremap(paddr, PAGE_SIZE, MEMREMAP_WB); > > (I don't undestand why there isn't a wrapper for this ... > Looks like we're doing something unexpected.) Do you mean a wrapper for getting the pfn/paddr? >> + if (!table) { >> + up_read(¤t->mm->mmap_sem); >> + return -EFAULT; >> + } >> + ret = CMPXCHG(&table[index], orig_pte, new_pte); >> + memunmap(table); >> + up_read(¤t->mm->mmap_sem); >> + } >> >> return (ret != orig_pte); >> } >> -- >> 2.7.4 I'll submit a v2 version soon. Amazon Development Center Germany GmbH Berlin - Dresden - Aachen main office: Krausenstr. 38, 10117 Berlin Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger Ust-ID: DE289237879 Eingetragen am Amtsgericht Charlottenburg HRB 149173 B
