Re: [RFC PATCH v2 28/69] KVM: Add per-VM flag to mark read-only memory as unsupported

[Brijesh Singh]

On 7/6/21 9:03 AM, Paolo Bonzini wrote:
> On 03/07/21 00:04, isaku.yamahata@xxxxxxxxx wrote:
> > From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> >
> > Add a flag for TDX to flag RO memory as unsupported and propagate it to
> > KVM_MEM_READONLY to allow reporting RO memory as unsupported on a per-VM
> > basis.  TDX1 doesn't expose permission bits to the VMM in the SEPT
> > tables, i.e. doesn't support read-only private memory.
> >
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > Co-developed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > ---
> >   arch/x86/kvm/x86.c       | 4 +++-
> >   include/linux/kvm_host.h | 4 ++++
> >   virt/kvm/kvm_main.c      | 8 +++++---
> >   3 files changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index cd9407982366..87212d7563ae 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3897,7 +3897,6 @@ int kvm_vm_ioctl_check_extension(struct kvm 
> > *kvm, long ext)
> >       case KVM_CAP_ASYNC_PF_INT:
> >       case KVM_CAP_GET_TSC_KHZ:
> >       case KVM_CAP_KVMCLOCK_CTRL:
> > -    case KVM_CAP_READONLY_MEM:
> >       case KVM_CAP_HYPERV_TIME:
> >       case KVM_CAP_IOAPIC_POLARITY_IGNORED:
> >       case KVM_CAP_TSC_DEADLINE_TIMER:
> > @@ -4009,6 +4008,9 @@ int kvm_vm_ioctl_check_extension(struct kvm 
> > *kvm, long ext)
> >           if (static_call(kvm_x86_is_vm_type_supported)(KVM_X86_TDX_VM))
> >               r |= BIT(KVM_X86_TDX_VM);
> >           break;
> > +    case KVM_CAP_READONLY_MEM:
> > +        r = kvm && kvm->readonly_mem_unsupported ? 0 : 1;
> > +        break;
> >       default:
> >           break;
> >       }
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index ddd4d0f68cdf..7ee7104b4b59 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -597,6 +597,10 @@ struct kvm {
> >       unsigned int max_halt_poll_ns;
> >       u32 dirty_ring_size;
> > +#ifdef __KVM_HAVE_READONLY_MEM
> > +    bool readonly_mem_unsupported;
> > +#endif
> > +
> >       bool vm_bugged;
> >   };
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 52d40ea75749..63d0c2833913 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1258,12 +1258,14 @@ static void update_memslots(struct 
> > kvm_memslots *slots,
> >       }
> >   }
> > -static int check_memory_region_flags(const struct 
> > kvm_userspace_memory_region *mem)
> > +static int check_memory_region_flags(struct kvm *kvm,
> > +                     const struct kvm_userspace_memory_region *mem)
> >   {
> >       u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
> >   #ifdef __KVM_HAVE_READONLY_MEM
> > -    valid_flags |= KVM_MEM_READONLY;
> > +    if (!kvm->readonly_mem_unsupported)
> > +        valid_flags |= KVM_MEM_READONLY;
> >   #endif
> >       if (mem->flags & ~valid_flags)
> > @@ -1436,7 +1438,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
> >       int as_id, id;
> >       int r;
> > -    r = check_memory_region_flags(mem);
> > +    r = check_memory_region_flags(kvm, mem);
> >       if (r)
> >           return r;
>
> For all these flags, which of these limitations will be common to SEV-ES 
> and SEV-SNP (ExtINT injection, MCE injection, changing TSC, read-only 
> memory, dirty logging)?  Would it make sense to use vm_type instead of 
> all of them?  I guess this also guides the choice of whether to use a 
> single vm-type for TDX and SEV-SNP or two.  Probably two is better, and 
> there can be static inline bool functions to derive the support flags 
> from the vm-type.

The SEV-ES does not need any of these flags. However, with SEV-SNP, we 
may able use the ExtINT injection.

-Brijesh