Re: [RFC PATCH v2 1/1] platform-msi: Add platform check for subdevice irq domain

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi,

On 1/7/21 3:16 PM, Leon Romanovsky wrote:
> On Thu, Jan 07, 2021 at 06:55:16AM +0000, Tian, Kevin wrote:
>>> From: Leon Romanovsky <leon@kernel.org>
>>> Sent: Thursday, January 7, 2021 2:09 PM
>>>
>>> On Thu, Jan 07, 2021 at 02:04:29AM +0000, Tian, Kevin wrote:
>>>>> From: Leon Romanovsky <leon@kernel.org>
>>>>> Sent: Thursday, January 7, 2021 12:02 AM
>>>>>
>>>>> On Wed, Jan 06, 2021 at 11:23:39AM -0400, Jason Gunthorpe wrote:
>>>>>> On Wed, Jan 06, 2021 at 12:40:17PM +0200, Leon Romanovsky wrote:
>>>>>>
>>>>>>> I asked what will you do when QEMU will gain needed functionality?
>>>>>>> Will you remove QEMU from this list? If yes, how such "new" kernel
>>> will
>>>>>>> work on old QEMU versions?
>>>>>>
>>>>>> The needed functionality is some VMM hypercall, so presumably new
>>>>>> kernels that support calling this hypercall will be able to discover
>>>>>> if the VMM hypercall exists and if so superceed this entire check.
>>>>>
>>>>> Let's not speculate, do we have well-known path?
>>>>> Will such patch be taken to stable@/distros?
>>>>>
>>>>
>>>> There are two functions introduced in this patch. One is to detect whether
>>>> running on bare metal or in a virtual machine. The other is for deciding
>>>> whether the platform supports ims. Currently the two are identical because
>>>> ims is supported only on bare metal at current stage. In the future it will
>>> look
>>>> like below when ims can be enabled in a VM:
>>>>
>>>> bool arch_support_pci_device_ims(struct pci_dev *pdev)
>>>> {
>>>> 	return on_bare_metal() || hypercall_irq_domain_supported();
>>>> }
>>>>
>>>> The VMM vendor list is for on_bare_metal, and suppose a vendor will
>>>> never be removed once being added to the list since the fact of running
>>>> in a VM never changes, regardless of whether this hypervisor supports
>>>> extra VMM hypercalls.
>>>
>>> This is what I imagined, this list will be forever, and this worries me.
>>>
>>> I don't know if it is true or not, but guess that at least Oracle and
>>> Microsoft bare metal devices and VMs will have same DMI_SYS_VENDOR.
>>
>> It's true. David Woodhouse also said it's the case for Amazon EC2 instances.
>>
>>>
>>> It means that this on_bare_metal() function won't work reliably in many
>>> cases. Also being part of include/linux/msi.h, at some point of time,
>>> this function will be picked by the users outside for the non-IMS cases.
>>>
>>> I didn't even mention custom forks of QEMU which are prohibited to change
>>> DMI_SYS_VENDOR and private clouds with custom solutions.
>>
>> In this case the private QEMU forks are encouraged to set CPUID (X86_
>> FEATURE_HYPERVISOR) if they do plan to adopt a different vendor name.
> 
> Does QEMU set this bit when it runs in host-passthrough CPU model?
> 
>>
>>>
>>> The current array makes DMI_SYS_VENDOR interface as some sort of ABI. If
>>> in the future,
>>> the QEMU will decide to use more hipster name, for example "qEmU", this
>>> function
>>> won't work.
>>>
>>> I'm aware that DMI_SYS_VENDOR is used heavily in the kernel code and
>>> various names for the same company are good example how not reliable it.
>>>
>>> The most hilarious example is "Dell/Dell Inc./Dell Inc/Dell Computer
>>> Corporation/Dell Computer",
>>> but other companies are not far from them.
>>>
>>> Luckily enough, this identification is used for hardware product that
>>> was released to the market and their name will be stable for that
>>> specific model. It is not the case here where we need to ensure future
>>> compatibility too (old kernel on new VM emulator).
>>>
>>> I'm not in position to say yes or no to this patch and don't have plans to do it.
>>> Just expressing my feeling that this solution is too hacky for my taste.
>>>
>>
>> I agree with your worries and solely relying on DMI_SYS_VENDOR is
>> definitely too hacky. In previous discussions with Thomas there is no
>> elegant way to handle this situation. It has to be a heuristic approach.
>> First we hope the CPUID bit is set properly in most cases thus is checked
>> first. Then other heuristics can be made for the remaining cases. DMI_
>> SYS_VENDOR is the first hint and more can be added later. For example,
>> when IOMMU is present there is vendor specific way to detect whether
>> it's real or virtual. Dave also mentioned some BIOS flag to indicate a
>> virtual machine. Now probably the real question here is whether people
>> are OK with CPUID+DMI_SYS_VENDOR combo check for now (and grow
>> it later) or prefer to having all identified heuristics so far in-place together...
> 
> IMHO, it should be as much as possible close to the end result.

Okay! This seems to be a right way to go.

The SMBIOS defines a 'virtual machine' bit in the BIOS characteristics
extension byte. It could be used as a possible way.

In order to support emulated IOMMU for fully virtualized guest, the
iommu vendors defined methods to distinguish between bare metal and VMM
(caching mode in VT-d for example).

I will go ahead with adding above two methods before checking the block
list.

Best regards,
baolu