Re: [PATCH v5 2/2] kvm: x86: hyperv: guest->host event signaling via eventfd

[David Hildenbrand <david@redhat.com>]

On 13.12.2017 12:51, David Hildenbrand wrote:
> On 13.12.2017 12:46, David Hildenbrand wrote:
>> On 13.12.2017 12:44, Roman Kagan wrote:
>>> On Wed, Dec 13, 2017 at 10:55:02AM +0100, David Hildenbrand wrote:
>>>>
>>>>> +static u16 kvm_hvcall_signal_event(struct kvm_vcpu *vcpu, bool fast, u64 param)
>>>>> +{
>>>>> +	u16 ret;
>>>>> +	u32 conn_id, flag_no;
>>>>> +	int idx;
>>>>> +	struct eventfd_ctx *eventfd;
>>>>> +
>>>>> +	if (unlikely(!fast)) {
>>>>> +		gpa_t gpa = param;
>>>>> +
>>>>> +		if ((gpa & (__alignof__(param) - 1)) ||
>>>>> +		    offset_in_page(gpa) + sizeof(param) > PAGE_SIZE)
>>>>> +			return HV_STATUS_INVALID_ALIGNMENT;
>>>>> +
>>>>> +		idx = srcu_read_lock(&vcpu->kvm->srcu);
>>>>> +		ret = kvm_vcpu_read_guest(vcpu, gpa, &param, sizeof(param));
>>>>> +		srcu_read_unlock(&vcpu->kvm->srcu, idx);
>>>>> +
>>>>> +		if (ret < 0)
>>>>> +			return HV_STATUS_INSUFFICIENT_MEMORY;
>>>>
>>>> "... event flags do not require any buffer allocation or queuing within
>>>> the hypervisor, so HvSignalEvent will never fail due to insufficient
>>>> resources."
>>>
>>> Ouch.  I wonder what else to map -EFAULT to then...  Looks like
>>> HV_STATUS_INVALID_ALIGNMENT is the most appropriate (as in "The input or
>>> output GPA pointer is not within the bounds of the GPA space.")
>>>
>>> Thanks,
>>> Roman.
>>>
>>
>> It is likely happen rearelyn, but usually if we are out of memory we
>> should report to user space. (chances that we won't be able to continue
>> for longer either way are very high)
>>
> (I implied -ENOMEM is translated to -EFAULT) it can of course happen if
> the address is wrong.
> 
> Also wonder how to deal with that. Could it be that we even have to
> trigger a page fault in the guest?
> 


The hypervisor will validate that the calling partition can read from
the input page before executing the
requested hypercall. This validation consists of two checks: the
specified GPA is mapped and the GPA is
marked readable. If either of these tests fails, the hypervisor
generates a memory intercept message.

So memory intercept message is the right thing to do I guess?

-- 

Thanks,

David / dhildenb