Re: [PATCH v2 2/3] x86/kvm/hyper-v: remove stale entries from vec_bitmap/auto_eoi_bitmap on vector change

Vitaly Kuznetsov
2018-03-09T15:21:25Z

Radim Krčmář <rkrcmar@redhat.com> writes:

> 2018-03-01 15:15+0100, Vitaly Kuznetsov:
>> When a new vector is written to SINx we update vec_bitmap/auto_eoi_bitmap
>> but we forget to remove old vector from these masks (in case it is not
>> present in some other SINTx).
>>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
>> ---
>> arch/x86/include/uapi/asm/hyperv.h | 2 ++
>> arch/x86/kvm/hyperv.c | 32 ++++++++++++++++++++++----------
>> 2 files changed, 24 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/x86/include/uapi/asm/hyperv.h b/arch/x86/include/uapi/asm/hyperv.h
>> index 197c2e6c7376..62c778a303a1 100644
>> --- a/arch/x86/include/uapi/asm/hyperv.h
>> +++ b/arch/x86/include/uapi/asm/hyperv.h
>> @@ -318,6 +318,8 @@ typedef struct _HV_REFERENCE_TSC_PAGE {
>> #define HV_SYNIC_SINT_COUNT (16)
>> /* Define the expected SynIC version. */
>> #define HV_SYNIC_VERSION_1 (0x1)
>> +/* Valid SynIC vectors are 16-255. */
>> +#define HV_SYNIC_FIRST_VALID_VECTOR (16)
>>
>> #define HV_SYNIC_CONTROL_ENABLE (1ULL << 0)
>> #define HV_SYNIC_SIMP_ENABLE (1ULL << 0)
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index 05f414525538..6d14f808145d 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -74,13 +74,30 @@ static bool synic_has_vector_auto_eoi(struct kvm_vcpu_hv_synic *synic,
>> return false;
>> }
>>
>> +static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
>> + int vector)
>> +{
>> + if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
>> + return;
>> +
>> + if (synic_has_vector_connected(synic, vector))
>> + __set_bit(vector, synic->vec_bitmap);
>> + else
>> + __clear_bit(vector, synic->vec_bitmap);
>> +
>> + if (synic_has_vector_auto_eoi(synic, vector))
>> + __set_bit(vector, synic->auto_eoi_bitmap);
>> + else
>> + __clear_bit(vector, synic->auto_eoi_bitmap);
>> +}
>> +
>> static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint,
>> u64 data, bool host)
>> {
>> - int vector;
>> + int vector, old_vector;
>>
>> vector = data & HV_SYNIC_SINT_VECTOR_MASK;
>> - if (vector < 16 && !host)
>> + if (vector < HV_SYNIC_FIRST_VALID_VECTOR && !host)
>> return 1;
>> /*
>> * Guest may configure multiple SINTs to use the same vector, so
>> @@ -88,18 +105,13 @@ static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint,
>> * bitmap of vectors with auto-eoi behavior. The bitmaps are
>> * updated here, and atomically queried on fast paths.
>> */
>> + old_vector = synic_read_sint(synic, sint) & HV_SYNIC_SINT_VECTOR_MASK;
>>
>> atomic64_set(&synic->sint[sint], data);
>>
>> - if (synic_has_vector_connected(synic, vector))
>> - __set_bit(vector, synic->vec_bitmap);
>> - else
>> - __clear_bit(vector, synic->vec_bitmap);
>> + synic_update_vector(synic, old_vector);
>>
>> - if (synic_has_vector_auto_eoi(synic, vector))
>> - __set_bit(vector, synic->auto_eoi_bitmap);
>> - else
>> - __clear_bit(vector, synic->auto_eoi_bitmap);
>> + synic_update_vector(synic, vector);
>
> This looks like it solves the problem when we get two SINTs with the
> same vector back-to-back , but shouldn't these bits really be cleared on
> EOI (either auto or manual)?

Hmm,

I was trying to address the following issue: guest programs SynIC's
SINTx with some vector but later re-programs it with a different
one. Without the patch synic->vec_bitmap and synic->auto_eoi_bitmap keep
stale data. If there's no concurrent interrupt than we're safe, but what
happens if there is one...

kvm_hv_synic_send_eoi() already goes through all SINTx but we already
updated vector so it won't find any. We could've added something like
'old_vector' but what if the request with this vector came _after_ we
re-programed SynIC (and, so, it wasn't meant to be serviced by SynIC?)?

--
Vitaly