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SubjectRe: [PATCH v6 11/12] KVM: introduce readonly memslot
On 2012-08-21 05:02, 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
>
> We introduce readonly memory region to map ROM/ROMD to the guest, read access
> is happy for readonly memslot, write access on readonly memslot will cause
> KVM_EXIT_MMIO exit
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> ---
> Documentation/virtual/kvm/api.txt | 10 +++-
> arch/x86/include/asm/kvm.h | 1 +
> arch/x86/kvm/mmu.c | 9 ++++
> arch/x86/kvm/x86.c | 1 +
> include/linux/kvm.h | 6 ++-
> include/linux/kvm_host.h | 7 +--
> virt/kvm/kvm_main.c | 96 ++++++++++++++++++++++++++++++-------
> 7 files changed, 102 insertions(+), 28 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index bf33aaa..b91bfd4 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 << 0)
> +#define KVM_MEM_READONLY (1UL << 1)
>
> 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 +874,12 @@ 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 flag, 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_READONLY when the
> +KVM_CAP_READONLY_MEM capability, it indicates the guest memory is read-only,
> +that means, guest is only allowed to read it.

The last sentence requires some refactoring. :) How about: "The
KVM_CAP_READONLY_MEM capability indicates the availability of the
KVM_MEM_READONLY flag. When this flag is set for a memory region, KVM
only allows read accesses."

> Writes will be posted to
> +userspace as KVM_EXIT_MMIO exits.
>
> When the KVM_CAP_SYNC_MMU capability, changes in the backing of the memory
> region are automatically reflected into the guest. For example, an mmap()
> diff --git a/arch/x86/include/asm/kvm.h b/arch/x86/include/asm/kvm.h
> index 246617e..521bf25 100644
> --- a/arch/x86/include/asm/kvm.h
> +++ b/arch/x86/include/asm/kvm.h
> @@ -25,6 +25,7 @@
> #define __KVM_HAVE_DEBUGREGS
> #define __KVM_HAVE_XSAVE
> #define __KVM_HAVE_XCRS
> +#define __KVM_HAVE_READONLY_MEM
>
> /* Architectural interrupt line count. */
> #define KVM_NR_INTERRUPTS 256
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 5548971..8e312a2 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2647,6 +2647,15 @@ static void kvm_send_hwpoison_signal(unsigned long address, struct task_struct *
>
> static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, pfn_t pfn)
> {
> + /*
> + * Do not cache the mmio info caused by writing the readonly gfn
> + * into the spte otherwise read access on readonly gfn also can
> + * caused mmio page fault and treat it as mmio access.
> + * Return 1 to tell kvm to emulate it.
> + */
> + if (pfn == KVM_PFN_ERR_RO_FAULT)
> + return 1;
> +
> if (pfn == KVM_PFN_ERR_HWPOISON) {
> kvm_send_hwpoison_signal(gfn_to_hva(vcpu->kvm, gfn), current);
> return 0;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 704680d..42bbf41 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2175,6 +2175,7 @@ int kvm_dev_ioctl_check_extension(long ext)
> case KVM_CAP_GET_TSC_KHZ:
> case KVM_CAP_PCI_2_3:
> case KVM_CAP_KVMCLOCK_CTRL:
> + case KVM_CAP_READONLY_MEM:
> r = 1;
> break;
> case KVM_CAP_COALESCED_MMIO:
> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> index 2de335d..d808694 100644
> --- a/include/linux/kvm.h
> +++ b/include/linux/kvm.h
> @@ -106,7 +106,8 @@ struct kvm_userspace_memory_region {
> * other bits are reserved for kvm internal use which are defined in
> * include/linux/kvm_host.h.
> */
> -#define KVM_MEM_LOG_DIRTY_PAGES 1UL
> +#define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
> +#define KVM_MEM_READONLY (1UL << 1)
>
> /* for KVM_IRQ_LINE */
> struct kvm_irq_level {
> @@ -621,6 +622,9 @@ struct kvm_ppc_smmu_info {
> #define KVM_CAP_PPC_GET_SMMU_INFO 78
> #define KVM_CAP_S390_COW 79
> #define KVM_CAP_PPC_ALLOC_HTAB 80
> +#ifdef __KVM_HAVE_READONLY_MEM
> +#define KVM_CAP_READONLY_MEM 81
> +#endif

See the discussion on the userspace patches: The CAP, as userspace API,
should really be defined unconditionally, kernel code should depend on
__KVM_HAVE_READONLY_MEM or a to-be-introduced CONFIG_KVM_HAVE_xxx
switch. This allows for cleaner userspace code.

Jan

--
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux


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