Re: [PATCH 5/6] kvm/i386: Add support for user space MSR filtering

[Siddharth Chandrasekaran]

On Tue, Jun 08, 2021 at 10:48:53AM +0200, Alexander Graf wrote:
> On 24.05.21 22:01, Siddharth Chandrasekaran wrote:
> > Check and enable user space MSR filtering capability and handle new exit
> > reason KVM_EXIT_X86_WRMSR. This will be used in a follow up patch to
> > implement hyper-v overlay pages.
> > 
> > Signed-off-by: Siddharth Chandrasekaran <sidcha@amazon.de>
> 
> This patch will break bisection, because we're no longer handling the writes
> in kernel space after this, but we also don't have user space handling
> available yet, right? It might be better to move all logic in this patch
> that sets up the filter for Hyper-V MSRs into the next one.

Yes, that's correct. I'll just bounce back all reads/writes to KVM. That
should maintain the existing behaviour.

> > ---
> >   target/i386/kvm/kvm.c | 72 +++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 72 insertions(+)
> > 
> > diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> > index 362f04ab3f..3591f8cecc 100644
> > --- a/target/i386/kvm/kvm.c
> > +++ b/target/i386/kvm/kvm.c
> > @@ -117,6 +117,8 @@ static bool has_msr_ucode_rev;
> >   static bool has_msr_vmx_procbased_ctls2;
> >   static bool has_msr_perf_capabs;
> >   static bool has_msr_pkrs;
> > +static bool has_msr_filtering;
> > +static bool msr_filters_active;
> >   static uint32_t has_architectural_pmu_version;
> >   static uint32_t num_architectural_pmu_gp_counters;
> > @@ -2138,6 +2140,57 @@ static void register_smram_listener(Notifier *n, 
> > void *unused)
> >                                    &smram_address_space, 1);
> >   }
> > +static void kvm_set_msr_filter_range(struct kvm_msr_filter_range *range, 
> > uint32_t flags,
> > +                                     uint32_t base, uint32_t nmsrs, ...)
> > +{
> > +    int i, filter_to_userspace;
> > +    va_list ap;
> > +
> > +    range->flags = flags;
> > +    range->nmsrs = nmsrs;
> > +    range->base = base;
> > +
> > +    va_start(ap, nmsrs);
> > +    for (i = 0; i < nmsrs; i++) {
> > +        filter_to_userspace = va_arg(ap, int);
> > +        if (!filter_to_userspace) {
> > +            range->bitmap[i / 8] = 1 << (i % 8);
> > +        }
> > +    }
> > +    va_end(ap);
> > +}
> > +
> > +static int kvm_set_msr_filters(KVMState *s)
> > +{
> > +    int r, nmsrs, nfilt = 0, bitmap_pos = 0;
> > +    struct kvm_msr_filter filter = { };
> > +    struct kvm_msr_filter_range *range;
> > +    uint8_t bitmap_buf[KVM_MSR_FILTER_MAX_RANGES * 8] = {0};
> > +
> > +    filter.flags = KVM_MSR_FILTER_DEFAULT_ALLOW;
> > +
> > +    if (has_hyperv) {
> > +        /* Hyper-V overlay page MSRs */
> 
> I think you want to extend this comment and indicate in a human readable
> form that you set the filter on WRMSR to trap HV_X64_MSR_GUEST_OS_ID and
> HV_X64_MSR_HYPERCALL into user space here.

Sure.

> > +        nmsrs = 2;
> > +        range = &filter.ranges[nfilt++];
> > +        range->bitmap = &bitmap_buf[bitmap_pos];
> > +        kvm_set_msr_filter_range(range, KVM_MSR_FILTER_WRITE,
> > +                                 HV_X64_MSR_GUEST_OS_ID, nmsrs,
> > +                                 true, /* HV_X64_MSR_GUEST_OS_ID */
> > +                                 true  /* HV_X64_MSR_HYPERCALL */);
> > +        bitmap_pos += ROUND_UP(nmsrs, 8) / 8;
> > +        assert(bitmap_pos < sizeof(bitmap_buf));
> > +    }
> > +
> > +    r = kvm_vm_ioctl(s, KVM_X86_SET_MSR_FILTER, &filter);
> > +    if (r != 0) {
> > +        error_report("kvm: failed to set MSR filters");
> > +        return -1;
> > +    }
> > +
> > +    return 0;
> > +}
> > +
> >   int kvm_arch_init(MachineState *ms, KVMState *s)
> >   {
> >       uint64_t identity_base = 0xfffbc000;
> > @@ -2269,6 +2322,17 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
> >           }
> >       }
> > +    has_msr_filtering = kvm_check_extension(s, KVM_CAP_X86_USER_SPACE_MSR) 
> > &&
> > +                        kvm_check_extension(s, KVM_CAP_X86_MSR_FILTER);
> > +    if (has_msr_filtering) {
> > +        ret = kvm_vm_enable_cap(s, KVM_CAP_X86_USER_SPACE_MSR, 0,
> > +                                KVM_MSR_EXIT_REASON_FILTER);
> > +        if (ret == 0) {
> > +            ret = kvm_set_msr_filters(s);
> > +            msr_filters_active = (ret == 0);
> > +        }
> > +    }
> > +
> >       return 0;
> >   }
> > @@ -4542,6 +4606,11 @@ static bool host_supports_vmx(void)
> >       return ecx & CPUID_EXT_VMX;
> >   }
> > +static int kvm_handle_wrmsr(X86CPU *cpu, struct kvm_run *run)
> > +{
> > +    return 0;
> 
> The default handler should always set run->msr.error = 1 to mimic the
> existing behavior.

Will do, thanks.

> > +}
> > +
> >   #define VMX_INVALID_GUEST_STATE 0x80000021
> >   int kvm_arch_handle_exit(CPUState *cs, struct kvm_run *run)
> > @@ -4600,6 +4669,9 @@ int kvm_arch_handle_exit(CPUState *cs, struct kvm_run 
> > *run)
> >           ioapic_eoi_broadcast(run->eoi.vector);
> >           ret = 0;
> >           break;
> > +    case KVM_EXIT_X86_WRMSR:
> > +        ret = kvm_handle_wrmsr(cpu, run);
> 
> Please provide a default RDMSR handler as well here.

Ack.

~ Sid.



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