Re: [PATCH v2 15/15] Drivers: hv: Add modules to expose /dev/mshv to VMMs running on Hyper-V

[Nuno Das Neves <nunodasneves@linux.microsoft.com>]

On 8/23/2023 12:40 AM, Saurabh Singh Sengar wrote:
> 
> 
>> -----Original Message-----
>> From: Nuno Das Neves <nunodasneves@linux.microsoft.com>
>> Sent: Wednesday, August 23, 2023 1:49 AM
>> To: Saurabh Singh Sengar <ssengar@microsoft.com>; linux-
>> hyperv@vger.kernel.org; linux-kernel@vger.kernel.org; x86@kernel.org; linux-
>> arm-kernel@lists.infradead.org; linux-arch@vger.kernel.org
>> Cc: patches@lists.linux.dev; Michael Kelley (LINUX)
>> <mikelley@microsoft.com>; KY Srinivasan <kys@microsoft.com>;
>> wei.liu@kernel.org; Haiyang Zhang <haiyangz@microsoft.com>; Dexuan Cui
>> <decui@microsoft.com>; apais@linux.microsoft.com; Tianyu Lan
>> <Tianyu.Lan@microsoft.com>; ssengar@linux.microsoft.com; MUKESH
>> RATHOR <mukeshrathor@microsoft.com>; stanislav.kinsburskiy@gmail.com;
>> jinankjain@linux.microsoft.com; vkuznets <vkuznets@redhat.com>;
>> tglx@linutronix.de; mingo@redhat.com; bp@alien8.de;
>> dave.hansen@linux.intel.com; hpa@zytor.com; will@kernel.org;
>> catalin.marinas@arm.com
>> Subject: Re: [PATCH v2 15/15] Drivers: hv: Add modules to expose /dev/mshv
>> to VMMs running on Hyper-V
>>
>> On 8/19/2023 10:19 PM, Saurabh Singh Sengar wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Nuno Das Neves <nunodasneves@linux.microsoft.com>
>>>> Sent: Saturday, August 19, 2023 12:30 AM
>>>> To: Saurabh Singh Sengar <ssengar@microsoft.com>; linux-
>>>> hyperv@vger.kernel.org; linux-kernel@vger.kernel.org; x86@kernel.org;
>>>> linux- arm-kernel@lists.infradead.org; linux-arch@vger.kernel.org
>>>> Cc: patches@lists.linux.dev; Michael Kelley (LINUX)
>>>> <mikelley@microsoft.com>; KY Srinivasan <kys@microsoft.com>;
>>>> wei.liu@kernel.org; Haiyang Zhang <haiyangz@microsoft.com>; Dexuan
>>>> Cui <decui@microsoft.com>; apais@linux.microsoft.com; Tianyu Lan
>>>> <Tianyu.Lan@microsoft.com>; ssengar@linux.microsoft.com; MUKESH
>>>> RATHOR <mukeshrathor@microsoft.com>;
>> stanislav.kinsburskiy@gmail.com;
>>>> jinankjain@linux.microsoft.com; vkuznets <vkuznets@redhat.com>;
>>>> tglx@linutronix.de; mingo@redhat.com; bp@alien8.de;
>>>> dave.hansen@linux.intel.com; hpa@zytor.com; will@kernel.org;
>>>> catalin.marinas@arm.com
>>>> Subject: Re: [PATCH v2 15/15] Drivers: hv: Add modules to expose
>>>> /dev/mshv to VMMs running on Hyper-V
>>>>
>>>> On 8/18/2023 6:08 AM, Saurabh Singh Sengar wrote:
>>>>>> +
>>>>>> +config MSHV_VTL
>>>>>> +	tristate "Microsoft Hyper-V VTL driver"
>>>>>> +	depends on MSHV
>>>>>> +	select HYPERV_VTL_MODE
>>>>>> +	select TRANSPARENT_HUGEPAGE
>>>>>
>>>>> TRANSPARENT_HUGEPAGE can be avoided for now.
>>>>>
>>>>
>>>> I will remove it in the next version. Thanks.
>>>>>> +
>>>>>> +#define HV_GET_REGISTER_BATCH_SIZE	\
>>>>>> +	(HV_HYP_PAGE_SIZE / sizeof(union hv_register_value))
>>>>>> +#define HV_SET_REGISTER_BATCH_SIZE	\
>>>>>> +	((HV_HYP_PAGE_SIZE - sizeof(struct hv_input_set_vp_registers)) \
>>>>>> +		/ sizeof(struct hv_register_assoc))
>>>>>> +
>>>>>> +int hv_call_get_vp_registers(
>>>>>> +		u32 vp_index,
>>>>>> +		u64 partition_id,
>>>>>> +		u16 count,
>>>>>> +		union hv_input_vtl input_vtl,
>>>>>> +		struct hv_register_assoc *registers) {
>>>>>> +	struct hv_input_get_vp_registers *input_page;
>>>>>> +	union hv_register_value *output_page;
>>>>>> +	u16 completed = 0;
>>>>>> +	unsigned long remaining = count;
>>>>>> +	int rep_count, i;
>>>>>> +	u64 status;
>>>>>> +	unsigned long flags;
>>>>>> +
>>>>>> +	local_irq_save(flags);
>>>>>> +
>>>>>> +	input_page = *this_cpu_ptr(hyperv_pcpu_input_arg);
>>>>>> +	output_page = *this_cpu_ptr(hyperv_pcpu_output_arg);
>>>>>> +
>>>>>> +	input_page->partition_id = partition_id;
>>>>>> +	input_page->vp_index = vp_index;
>>>>>> +	input_page->input_vtl.as_uint8 = input_vtl.as_uint8;
>>>>>> +	input_page->rsvd_z8 = 0;
>>>>>> +	input_page->rsvd_z16 = 0;
>>>>>> +
>>>>>> +	while (remaining) {
>>>>>> +		rep_count = min(remaining, HV_GET_REGISTER_BATCH_SIZE);
>>>>>> +		for (i = 0; i < rep_count; ++i)
>>>>>> +			input_page->names[i] = registers[i].name;
>>>>>> +
>>>>>> +		status = hv_do_rep_hypercall(HVCALL_GET_VP_REGISTERS,
>>>>>> rep_count,
>>>>>> +					     0, input_page, output_page);
>>>>>
>>>>> Is there any possibility that count value is passed 0 by mistake ?
>>>>> In that case status will remain uninitialized.
>>>>>
>>>>
>>>> These lines ensure rep_count is never 0 here:
>>>>
>>>> 	while (remaining) {
>>>> 		rep_count = min(remaining, HV_GET_REGISTER_BATCH_SIZE);
>>>>
>>>> Remaining can't be 0 or the loop would exit, and
>>>> HV_GET_REGISTER_BATCH_SIZE is not 0, or we would never get any
>> registers.
>>>
>>> There is a parameter in this function "count". I was checking if there
>>> is any possibility that is passed as 0 by mistake ? this will lead to
>>> "remaining" value as 0 and loop will never execute. Which results using
>> "status" uninitialized later in the function.
>>>
>>>
>>
>> Ah ok, thanks! I will change it to return immediately in case count is 0.
> 
> Or you can initialize status with appropriate error value, either way is fine.
> Please consider fixing the same issue in hv_call_set_vp_registers as well.
> 

Thanks again - noted.