Re: [PATCH v6 12/14] KVM: x86: Introduce KVM_PAGE_ENC_BITMAP_RESET ioctl

[Steve Rutherford]

On Fri, Apr 10, 2020 at 1:16 PM Steve Rutherford <srutherford@xxxxxxxxxx> wrote:
>
> On Fri, Apr 10, 2020 at 11:14 AM Steve Rutherford
> <srutherford@xxxxxxxxxx> wrote:
> >
> > On Thu, Apr 9, 2020 at 6:34 PM Ashish Kalra <ashish.kalra@xxxxxxx> wrote:
> > >
> > > Hello Steve,
> > >
> > > On Thu, Apr 09, 2020 at 05:59:56PM -0700, Steve Rutherford wrote:
> > > > On Tue, Apr 7, 2020 at 6:52 PM Ashish Kalra <ashish.kalra@xxxxxxx> wrote:
> > > > >
> > > > > Hello Steve,
> > > > >
> > > > > On Tue, Apr 07, 2020 at 06:25:51PM -0700, Steve Rutherford wrote:
> > > > > > On Mon, Apr 6, 2020 at 11:53 AM Krish Sadhukhan
> > > > > > <krish.sadhukhan@xxxxxxxxxx> wrote:
> > > > > > >
> > > > > > >
> > > > > > > On 4/3/20 2:45 PM, Ashish Kalra wrote:
> > > > > > > > On Fri, Apr 03, 2020 at 02:14:23PM -0700, Krish Sadhukhan wrote:
> > > > > > > >> On 3/29/20 11:23 PM, Ashish Kalra wrote:
> > > > > > > >>> From: Ashish Kalra <ashish.kalra@xxxxxxx>
> > > > > > > >>>
> > > > > > > >>> This ioctl can be used by the application to reset the page
> > > > > > > >>> encryption bitmap managed by the KVM driver. A typical usage
> > > > > > > >>> for this ioctl is on VM reboot, on reboot, we must reinitialize
> > > > > > > >>> the bitmap.
> > > > > > > >>>
> > > > > > > >>> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> > > > > > > >>> ---
> > > > > > > >>>    Documentation/virt/kvm/api.rst  | 13 +++++++++++++
> > > > > > > >>>    arch/x86/include/asm/kvm_host.h |  1 +
> > > > > > > >>>    arch/x86/kvm/svm.c              | 16 ++++++++++++++++
> > > > > > > >>>    arch/x86/kvm/x86.c              |  6 ++++++
> > > > > > > >>>    include/uapi/linux/kvm.h        |  1 +
> > > > > > > >>>    5 files changed, 37 insertions(+)
> > > > > > > >>>
> > > > > > > >>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > > > > > > >>> index 4d1004a154f6..a11326ccc51d 100644
> > > > > > > >>> --- a/Documentation/virt/kvm/api.rst
> > > > > > > >>> +++ b/Documentation/virt/kvm/api.rst
> > > > > > > >>> @@ -4698,6 +4698,19 @@ During the guest live migration the outgoing guest exports its page encryption
> > > > > > > >>>    bitmap, the KVM_SET_PAGE_ENC_BITMAP can be used to build the page encryption
> > > > > > > >>>    bitmap for an incoming guest.
> > > > > > > >>> +4.127 KVM_PAGE_ENC_BITMAP_RESET (vm ioctl)
> > > > > > > >>> +-----------------------------------------
> > > > > > > >>> +
> > > > > > > >>> +:Capability: basic
> > > > > > > >>> +:Architectures: x86
> > > > > > > >>> +:Type: vm ioctl
> > > > > > > >>> +:Parameters: none
> > > > > > > >>> +:Returns: 0 on success, -1 on error
> > > > > > > >>> +
> > > > > > > >>> +The KVM_PAGE_ENC_BITMAP_RESET is used to reset the guest's page encryption
> > > > > > > >>> +bitmap during guest reboot and this is only done on the guest's boot vCPU.
> > > > > > > >>> +
> > > > > > > >>> +
> > > > > > > >>>    5. The kvm_run structure
> > > > > > > >>>    ========================
> > > > > > > >>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > > > > > > >>> index d30f770aaaea..a96ef6338cd2 100644
> > > > > > > >>> --- a/arch/x86/include/asm/kvm_host.h
> > > > > > > >>> +++ b/arch/x86/include/asm/kvm_host.h
> > > > > > > >>> @@ -1273,6 +1273,7 @@ struct kvm_x86_ops {
> > > > > > > >>>                             struct kvm_page_enc_bitmap *bmap);
> > > > > > > >>>     int (*set_page_enc_bitmap)(struct kvm *kvm,
> > > > > > > >>>                             struct kvm_page_enc_bitmap *bmap);
> > > > > > > >>> +   int (*reset_page_enc_bitmap)(struct kvm *kvm);
> > > > > > > >>>    };
> > > > > > > >>>    struct kvm_arch_async_pf {
> > > > > > > >>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > > > > > > >>> index 313343a43045..c99b0207a443 100644
> > > > > > > >>> --- a/arch/x86/kvm/svm.c
> > > > > > > >>> +++ b/arch/x86/kvm/svm.c
> > > > > > > >>> @@ -7797,6 +7797,21 @@ static int svm_set_page_enc_bitmap(struct kvm *kvm,
> > > > > > > >>>     return ret;
> > > > > > > >>>    }
> > > > > > > >>> +static int svm_reset_page_enc_bitmap(struct kvm *kvm)
> > > > > > > >>> +{
> > > > > > > >>> +   struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > > > > > > >>> +
> > > > > > > >>> +   if (!sev_guest(kvm))
> > > > > > > >>> +           return -ENOTTY;
> > > > > > > >>> +
> > > > > > > >>> +   mutex_lock(&kvm->lock);
> > > > > > > >>> +   /* by default all pages should be marked encrypted */
> > > > > > > >>> +   if (sev->page_enc_bmap_size)
> > > > > > > >>> +           bitmap_fill(sev->page_enc_bmap, sev->page_enc_bmap_size);
> > > > > > > >>> +   mutex_unlock(&kvm->lock);
> > > > > > > >>> +   return 0;
> > > > > > > >>> +}
> > > > > > > >>> +
> > > > > > > >>>    static int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
> > > > > > > >>>    {
> > > > > > > >>>     struct kvm_sev_cmd sev_cmd;
> > > > > > > >>> @@ -8203,6 +8218,7 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
> > > > > > > >>>     .page_enc_status_hc = svm_page_enc_status_hc,
> > > > > > > >>>     .get_page_enc_bitmap = svm_get_page_enc_bitmap,
> > > > > > > >>>     .set_page_enc_bitmap = svm_set_page_enc_bitmap,
> > > > > > > >>> +   .reset_page_enc_bitmap = svm_reset_page_enc_bitmap,
> > > > > > > >>
> > > > > > > >> We don't need to initialize the intel ops to NULL ? It's not initialized in
> > > > > > > >> the previous patch either.
> > > > > > > >>
> > > > > > > >>>    };
> > > > > > > > This struct is declared as "static storage", so won't the non-initialized
> > > > > > > > members be 0 ?
> > > > > > >
> > > > > > >
> > > > > > > Correct. Although, I see that 'nested_enable_evmcs' is explicitly
> > > > > > > initialized. We should maintain the convention, perhaps.
> > > > > > >
> > > > > > > >
> > > > > > > >>>    static int __init svm_init(void)
> > > > > > > >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > > > > >>> index 05e953b2ec61..2127ed937f53 100644
> > > > > > > >>> --- a/arch/x86/kvm/x86.c
> > > > > > > >>> +++ b/arch/x86/kvm/x86.c
> > > > > > > >>> @@ -5250,6 +5250,12 @@ long kvm_arch_vm_ioctl(struct file *filp,
> > > > > > > >>>                     r = kvm_x86_ops->set_page_enc_bitmap(kvm, &bitmap);
> > > > > > > >>>             break;
> > > > > > > >>>     }
> > > > > > > >>> +   case KVM_PAGE_ENC_BITMAP_RESET: {
> > > > > > > >>> +           r = -ENOTTY;
> > > > > > > >>> +           if (kvm_x86_ops->reset_page_enc_bitmap)
> > > > > > > >>> +                   r = kvm_x86_ops->reset_page_enc_bitmap(kvm);
> > > > > > > >>> +           break;
> > > > > > > >>> +   }
> > > > > > > >>>     default:
> > > > > > > >>>             r = -ENOTTY;
> > > > > > > >>>     }
> > > > > > > >>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > > > > > > >>> index b4b01d47e568..0884a581fc37 100644
> > > > > > > >>> --- a/include/uapi/linux/kvm.h
> > > > > > > >>> +++ b/include/uapi/linux/kvm.h
> > > > > > > >>> @@ -1490,6 +1490,7 @@ struct kvm_enc_region {
> > > > > > > >>>    #define KVM_GET_PAGE_ENC_BITMAP  _IOW(KVMIO, 0xc5, struct kvm_page_enc_bitmap)
> > > > > > > >>>    #define KVM_SET_PAGE_ENC_BITMAP  _IOW(KVMIO, 0xc6, struct kvm_page_enc_bitmap)
> > > > > > > >>> +#define KVM_PAGE_ENC_BITMAP_RESET  _IO(KVMIO, 0xc7)
> > > > > > > >>>    /* Secure Encrypted Virtualization command */
> > > > > > > >>>    enum sev_cmd_id {
> > > > > > > >> Reviewed-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>
> > > > > >
> > > > > >
> > > > > > Doesn't this overlap with the set ioctl? Yes, obviously, you have to
> > > > > > copy the new value down and do a bit more work, but I don't think
> > > > > > resetting the bitmap is going to be the bottleneck on reboot. Seems
> > > > > > excessive to add another ioctl for this.
> > > > >
> > > > > The set ioctl is generally available/provided for the incoming VM to setup
> > > > > the page encryption bitmap, this reset ioctl is meant for the source VM
> > > > > as a simple interface to reset the whole page encryption bitmap.
> > > > >
> > > > > Thanks,
> > > > > Ashish
> > > >
> > > >
> > > > Hey Ashish,
> > > >
> > > > These seem very overlapping. I think this API should be refactored a bit.
> > > >
> > > > 1) Use kvm_vm_ioctl_enable_cap to control whether or not this
> > > > hypercall (and related feature bit) is offered to the VM, and also the
> > > > size of the buffer.
> > >
> > > If you look at patch 13/14, i have added a new kvm para feature called
> > > "KVM_FEATURE_SEV_LIVE_MIGRATION" which indicates host support for SEV
> > > Live Migration and a new Custom MSR which the guest does a wrmsr to
> > > enable the Live Migration feature, so this is like the enable cap
> > > support.
> > >
> > > There are further extensions to this support i am adding, so patch 13/14
> > > of this patch-set is still being enhanced and will have full support
> > > when i repost next.
> > >
> > > > 2) Use set for manipulating values in the bitmap, including resetting
> > > > the bitmap. Set the bitmap pointer to null if you want to reset to all
> > > > 0xFFs. When the bitmap pointer is set, it should set the values to
> > > > exactly what is pointed at, instead of only clearing bits, as is done
> > > > currently.
> > >
> > > As i mentioned in my earlier email, the set api is supposed to be for
> > > the incoming VM, but if you really need to use it for the outgoing VM
> > > then it can be modified.
> > >
> > > > 3) Use get for fetching values from the kernel. Personally, I'd
> > > > require alignment of the base GFN to a multiple of 8 (but the number
> > > > of pages could be whatever), so you can just use a memcpy. Optionally,
> > > > you may want some way to tell userspace the size of the existing
> > > > buffer, so it can ensure that it can ask for the entire buffer without
> > > > having to track the size in usermode (not strictly necessary, but nice
> > > > to have since it ensures that there is only one place that has to
> > > > manage this value).
> > > >
> > > > If you want to expand or contract the bitmap, you can use enable cap
> > > > to adjust the size.
> > >
> > > As being discussed on the earlier mail thread, we are doing this
> > > dynamically now by computing the guest RAM size when the
> > > set_user_memory_region ioctl is invoked. I believe that should handle
> > > the hot-plug and hot-unplug events too, as any hot memory updates will
> > > need KVM memslots to be updated.
> > Ahh, sorry, forgot you mentioned this: yes this can work. Host needs
> > to be able to decide not to allocate, but this should be workable.
> > >
> > > > If you don't want to offer the hypercall to the guest, don't call the
> > > > enable cap.
> > > > This API avoids using up another ioctl. Ioctl space is somewhat
> > > > scarce. It also gives userspace fine grained control over the buffer,
> > > > so it can support both hot-plug and hot-unplug (or at the very least
> > > > it is not obviously incompatible with those). It also gives userspace
> > > > control over whether or not the feature is offered. The hypercall
> > > > isn't free, and being able to tell guests to not call when the host
> > > > wasn't going to migrate it anyway will be useful.
> > > >
> > >
> > > As i mentioned above, now the host indicates if it supports the Live
> > > Migration feature and the feature and the hypercall are only enabled on
> > > the host when the guest checks for this support and does a wrmsr() to
> > > enable the feature. Also the guest will not make the hypercall if the
> > > host does not indicate support for it.
> > If my read of those patches was correct, the host will always
> > advertise support for the hypercall. And the only bit controlling
> > whether or not the hypercall is advertised is essentially the kernel
> > version. You need to rollout a new kernel to disable the hypercall.
>
> Ahh, awesome, I see I misunderstood how the CPUID bits get passed
> through: usermode can still override them. Forgot about the back and
> forth for CPUID with usermode. My point about informing the guest
> kernel is clearly moot. The host still needs the ability to prevent
> allocations, but that is more minor. Maybe use a flag on the memslots
> directly?
On second thought: burning the memslot flag for 30mb per tb of VM
seems like a waste.