Re: [PATCH v10 9/9] KVM: Enable and expose KVM_MEM_PRIVATE

[Sean Christopherson]

On Tue, Jan 17, 2023, Chao Peng wrote:
> On Sat, Jan 14, 2023 at 12:01:01AM +0000, Sean Christopherson wrote:
> > On Fri, Dec 02, 2022, Chao Peng wrote:
> > > @@ -10357,6 +10364,12 @@ static int vcpu_enter_guest(struct kvm_vcpu 
> > > *vcpu)
> > >  
> > >           if (kvm_check_request(KVM_REQ_UPDATE_CPU_DIRTY_LOGGING, vcpu))
> > >                   static_call(kvm_x86_update_cpu_dirty_logging)(vcpu);
> > > +
> > > +         if (kvm_check_request(KVM_REQ_MEMORY_MCE, vcpu)) {
> > > +                 vcpu->run->exit_reason = KVM_EXIT_SHUTDOWN;
> > 
> > Synthesizing triple fault shutdown is not the right approach.  Even with 
> > TDX's
> > MCE "architecture" (heavy sarcasm), it's possible that host userspace and 
> > the
> > guest have a paravirt interface for handling memory errors without killing 
> > the
> > host.
> 
> Agree shutdown is not the correct choice. I see you made below change:
> 
> send_sig_mceerr(BUS_MCEERR_AR, (void __user *)hva, PAGE_SHIFT, current)
> 
> The MCE may happen in any thread than KVM thread, sending siginal to
> 'current' thread may not be the expected behavior.

This is already true today, e.g. a #MC in memory that is mapped into the guest 
can
be triggered by a host access.  Hrm, but in this case we actually have a KVM
instance, and we know that the #MC is relevant to the KVM instance, so I agree
that signaling 'current' is kludgy.

>  Also how userspace can tell is the MCE on the shared page or private page?
>  Do we care?

We care.  I was originally thinking we could require userspace to keep track of
things, but that's quite prescriptive and flawed, e.g. could race with 
conversions.

One option would be to KVM_EXIT_MEMORY_FAULT, and then wire up a generic (not 
x86
specific) KVM request to exit to userspace, e.g.

                /* KVM_EXIT_MEMORY_FAULT */
                struct {
#define KVM_MEMORY_EXIT_FLAG_PRIVATE    (1ULL << 3)
#define KVM_MEMORY_EXIT_FLAG_HW_ERROR   (1ULL << 4)
                        __u64 flags;
                        __u64 gpa;
                        __u64 size;
                } memory;

But I'm not sure that's the correct approach.  It kinda feels like we're 
reinventing
the wheel.  It seems like restrictedmem_get_page() _must_ be able to reject 
attempts
to get a poisoned page, i.e. restrictedmem_get_page() should yield 
KVM_PFN_ERR_HWPOISON.
Assuming that's the case, then I believe KVM simply needs to zap SPTEs in 
response
to an error notification in order to force vCPUs to fault on the poisoned page.

> > > +         return -EINVAL;
> > >   if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_MEM_SLOTS_NUM)
> > >           return -EINVAL;
> > >   if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
> > > @@ -2020,6 +2154,9 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > >           if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages)
> > >                   return -EINVAL;
> > >   } else { /* Modify an existing slot. */
> > > +         /* Private memslots are immutable, they can only be deleted. */
> > 
> > I'm 99% certain I suggested this, but if we're going to make these memslots
> > immutable, then we should straight up disallow dirty logging, otherwise 
> > we'll
> > end up with a bizarre uAPI.
> 
> But in my mind dirty logging will be needed in the very short time, when
> live migration gets supported?

Ya, but if/when live migration support is added, private memslots will no longer
be immutable as userspace will want to enable dirty logging only when a VM is
being migrated, i.e. something will need to change.

Given that it looks like we have clear line of sight to SEV+UPM guests, my
preference would be to allow toggling dirty logging from the get-go.  It doesn't
necessarily have to be in the first patch, e.g. KVM could initially reject
KVM_MEM_LOG_DIRTY_PAGES + KVM_MEM_PRIVATE and then add support separately to 
make
the series easier to review, test, and bisect.

static int check_memory_region_flags(struct kvm *kvm,
                                     const struct kvm_userspace_memory_region2 
*mem)
{
        u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;

        if (kvm_arch_has_private_mem(kvm) &&
            ~(mem->flags & KVM_MEM_LOG_DIRTY_PAGES))
                valid_flags |= KVM_MEM_PRIVATE;


        ...
}

> > > +         if (mem->flags & KVM_MEM_PRIVATE)
> > > +                 return -EINVAL;
> > >           if ((mem->userspace_addr != old->userspace_addr) ||
> > >               (npages != old->npages) ||
> > >               ((mem->flags ^ old->flags) & KVM_MEM_READONLY))
> > > @@ -2048,10 +2185,28 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > >   new->npages = npages;
> > >   new->flags = mem->flags;
> > >   new->userspace_addr = mem->userspace_addr;
> > > + if (mem->flags & KVM_MEM_PRIVATE) {
> > > +         new->restricted_file = fget(mem->restricted_fd);
> > > +         if (!new->restricted_file ||
> > > +             !file_is_restrictedmem(new->restricted_file)) {
> > > +                 r = -EINVAL;
> > > +                 goto out;
> > > +         }
> > > +         new->restricted_offset = mem->restricted_offset;
> 
> I see you changed slot->restricted_offset type from loff_t to gfn_t and
> used pgoff_t when doing the restrictedmem_bind/unbind(). Using page
> index is reasonable KVM internally and sounds simpler than loff_t. But
> we also need initialize it to page index here as well as changes in
> another two cases. This is needed when restricted_offset != 0.

Oof.  I'm pretty sure I completely missed that loff_t is used for byte offsets,
whereas pgoff_t is a frame index. 

Given that the restrictmem APIs take pgoff_t, I definitely think it makes sense
to the index, but I'm very tempted to store pgoff_t instead of gfn_t, and name
the field "index" to help connect the dots to the rest of kernel, where 
"pgoff_t index"
is quite common.

And looking at those bits again, we should wrap all of the restrictedmem fields
with CONFIG_KVM_PRIVATE_MEM.  It'll require minor tweaks to 
__kvm_set_memory_region(),
but I think will yield cleaner code (and internal APIs) overall.

And wrap the three fields in an anonymous struct?  E.g. this is a little more
versbose (restrictedmem instead restricted), but at first glance it doesn't seem
to cause widespared line length issues.

#ifdef CONFIG_KVM_PRIVATE_MEM
        struct {
                struct file *file;
                pgoff_t index;
                struct restrictedmem_notifier notifier;
        } restrictedmem;
#endif

> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 547b92215002..49e375e78f30 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2364,8 +2364,7 @@ static inline int kvm_restricted_mem_get_pfn(struct 
> kvm_memory_slot *slot,
>                                              gfn_t gfn, kvm_pfn_t *pfn,
>                                              int *order)
>  {
> -       pgoff_t index = gfn - slot->base_gfn +
> -                       (slot->restricted_offset >> PAGE_SHIFT);
> +       pgoff_t index = gfn - slot->base_gfn + slot->restricted_offset;
>         struct page *page;
>         int ret;
>  
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 01db35ddd5b3..7439bdcb0d04 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -935,7 +935,7 @@ static bool restrictedmem_range_is_valid(struct 
> kvm_memory_slot *slot,
>                                          pgoff_t start, pgoff_t end,
>                                          gfn_t *gfn_start, gfn_t *gfn_end)
>  {
> -       unsigned long base_pgoff = slot->restricted_offset >> PAGE_SHIFT;
> +       unsigned long base_pgoff = slot->restricted_offset;
>  
>         if (start > base_pgoff)
>                 *gfn_start = slot->base_gfn + start - base_pgoff;
> @@ -2275,7 +2275,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
>                         r = -EINVAL;
>                         goto out;
>                 }
> -               new->restricted_offset = mem->restricted_offset;
> +               new->restricted_offset = mem->restricted_offset >> PAGE_SHIFT;
>         }
>  
>         r = kvm_set_memslot(kvm, old, new, change);
> 
> Chao
> > > + }
> > > +
> > > + new->kvm = kvm;
> > 
> > Set this above, just so that the code flows better.