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Date: Thu, 24 May 2012 15:10:09 +0300
From: Avi Kivity <avi@...hat.com>
To: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC: Marcelo Tosatti <mtosatti@...hat.com>,
Gleb Natapov <gleb@...hat.com>,
LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: introduce readonly memory region
On 05/24/2012 12:24 PM, Xiao Guangrong wrote:
> In current code, if we map a readonly memory space from host to guest
> and the page is not currently mapped in the host, we will get a fault-pfn
> and async is not allowed, then the vm will crash
>
> Address Avi's idea, we introduce readonly memory region to map ROM/ROMD
> to the guest
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
> Documentation/virtual/kvm/api.txt | 9 +++++--
> include/linux/kvm.h | 5 ++-
> virt/kvm/kvm_main.c | 43 ++++++++++++++++++++++++++++++-------
> 3 files changed, 44 insertions(+), 13 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 9301266..e2a82c3 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -857,7 +857,8 @@ struct kvm_userspace_memory_region {
> };
>
> /* for kvm_memory_region::flags */
> -#define KVM_MEM_LOG_DIRTY_PAGES 1UL
> +#define KVM_MEM_LOG_DIRTY_PAGES 1UL
> +#define KVM_MEM_READ_ONLY (1UL << 2)
Bit 1 should be fine too, see below.
>
> This ioctl allows the user to create or modify a guest physical memory
> slot. When changing an existing slot, it may be moved in the guest
> @@ -873,9 +874,11 @@ It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr
> be identical. This allows large pages in the guest to be backed by large
> pages in the host.
>
> -The flags field supports just one flag, KVM_MEM_LOG_DIRTY_PAGES, which
> +The flags field supports two flags, KVM_MEM_LOG_DIRTY_PAGES, which
> instructs kvm to keep track of writes to memory within the slot. See
> -the KVM_GET_DIRTY_LOG ioctl.
> +the KVM_GET_DIRTY_LOG ioctl. Another flag is KVM_MEM_READ_ONLY, which
> +indicates the guest memory is read-only, that means, guest is only allowed
> +to read it.
+ Writes will be posted to userspace as KVM_EXIT_MMIO exits.
>
> /* for kvm_memory_region::flags */
> -#define KVM_MEM_LOG_DIRTY_PAGES 1UL
> -#define KVM_MEMSLOT_INVALID (1UL << 1)
> +#define KVM_MEM_LOG_DIRTY_PAGES 1UL
> +#define KVM_MEMSLOT_INVALID (1UL << 1)
> +#define KVM_MEM_READ_ONLY (1UL << 2)
KVM_MEMSLOT_INVALID is actually an internal symbol, not used by
userspace. Please move it to kvm_host.h.
I see that we don't check flags for validity. Please add a check that
we don't use undefined flags and return -EINVAL. Should be a separate
patch since we may want to backport it.
We need a KVM_CAP_ so userspace knows it can use the feature. Only x86
should respond to it now, until (or if) other archs are updated.
>
> +static bool vma_is_avalid(struct vm_area_struct *vma, bool write_fault)
s/avalid/valid/.
> +{
> + if (write_fault) {
> + if (unlikely(!(vma->vm_flags & VM_WRITE)))
> + return false;
> +
> + return true;
> + }
> +
> + if (unlikely(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))))
> + return false;
> +
Strange check. VM_EXEC doesn't concern us at all. Maybe we should
check for VM_READ always, and VM_WRITE for write faults.
> + return true;
> +}
> +
> static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr, bool atomic,
> bool *async, bool write_fault, bool *writable)
> {
> @@ -1076,7 +1103,6 @@ static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr, bool atomic,
>
> if (writable)
> *writable = write_fault;
> -
> if (async) {
> down_read(¤t->mm->mmap_sem);
> npages = get_user_page_nowait(current, current->mm,
> @@ -1123,8 +1149,9 @@ static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr, bool atomic,
> vma->vm_pgoff;
> BUG_ON(!kvm_is_mmio_pfn(pfn));
> } else {
> - if (async && (vma->vm_flags & VM_WRITE))
> + if (async && vma_is_avalid(vma, write_fault))
> *async = true;
> +
This checks based on the fault type, not memslot type. So we have the
risk of the pfn later used for writes?
> pfn = get_fault_pfn();
> }
> up_read(¤t->mm->mmap_sem);
> @@ -1148,7 +1175,7 @@ static pfn_t __gfn_to_pfn(struct kvm *kvm, gfn_t gfn, bool atomic, bool *async,
> if (async)
> *async = false;
>
> - addr = gfn_to_hva(kvm, gfn);
> + addr = gfn_to_hva_prot(kvm, gfn, write_fault);
> if (kvm_is_error_hva(addr)) {
> get_page(bad_page);
> return page_to_pfn(bad_page);
> @@ -1293,7 +1320,7 @@ int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
> int r;
> unsigned long addr;
>
> - addr = gfn_to_hva(kvm, gfn);
> + addr = gfn_to_hva_prot(kvm, gfn, false);
> if (kvm_is_error_hva(addr))
> return -EFAULT;
> r = __copy_from_user(data, (void __user *)addr + offset, len);
> @@ -1331,7 +1358,7 @@ int kvm_read_guest_atomic(struct kvm *kvm, gpa_t gpa, void *data,
> gfn_t gfn = gpa >> PAGE_SHIFT;
> int offset = offset_in_page(gpa);
>
> - addr = gfn_to_hva(kvm, gfn);
> + addr = gfn_to_hva_prot(kvm, gfn, false);
> if (kvm_is_error_hva(addr))
> return -EFAULT;
> pagefault_disable();
Surprised only those places.
How do we make sure a pfn obtained with write = false isn't later used
for writing?
--
error compiling committee.c: too many arguments to function
--
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