Re: [PATCH v11 22/26] virt: gunyah: Add proxy-scheduled vCPUs

[Elliot Berman]

On 4/17/2023 3:41 PM, Elliot Berman wrote:
> On 3/31/2023 7:27 AM, Alex Elder wrote:
> > On 3/3/23 7:06 PM, Elliot Berman wrote:
>
> [snip]
>
> > > diff --git a/include/uapi/linux/gunyah.h b/include/uapi/linux/gunyah.h
> > > index caeb3b3a3e9a..e52265fa5715 100644
> > > --- a/include/uapi/linux/gunyah.h
> > > +++ b/include/uapi/linux/gunyah.h
> > > @@ -62,8 +62,32 @@ struct gh_vm_dtb_config {
> > >   #define GH_VM_START        _IO(GH_IOCTL_TYPE, 0x3)
> > > +/**
> > > + * GH_FN_VCPU - create a vCPU instance to control a vCPU
> > > + *
> > > + * gh_fn_desc is filled with &struct gh_fn_vcpu_arg
> > > + *
> > > + * The vcpu type will register with the VM Manager to expect to control
> > > + * vCPU number `vcpu_id`. It returns a file descriptor allowing 
> > > interaction with
> > > + * the vCPU. See the Gunyah vCPU API description sections for 
> > > interacting with
> > > + * the Gunyah vCPU file descriptors.
> > > + *
> > > + * Return: file descriptor to manipulate the vcpu. See GH_VCPU_* ioctls
> > > + */
> > > +#define GH_FN_VCPU         1
> >
> > I think you should define GH_VN_VCPU, GN_FN_IRQFD, and GN_FN_IOEVENTFD
> > in an enumerated type.  Each has a type associated with it, and you can
> > add the explanation for the function in the kernel-doc comments above
> > thosse type definitions.
>
> I'd like to enumify the GH_FN_* macros, but one challenge I'm facing is 
> that it breaks the module alias implementation in patch 19.
>
> MODULE_ALIAS("ghfunc:"__stringify(_type))
>
> When the GH_FN_* are regular preprocessor macros backed by an integer, 
> the preprocessor will make the module alias ghfunc:0 (or ghfunc:1, etc). 
> This works well because I can do
>
> request_module("ghfunc:%d", type);
>
> If the function hasn't been registered and then gunyah_vcpu.ko gets 
> loaded automatically.
>
> With enum, compiler knows the value of GH_FN_VCPU and preprocessor will 
> make the module alias like ghfunc:GH_FN_VCPU.

I still like the idea of having enum for documentation and clarity. I 
noticed that nfnetlink.h saw the same problem for NFNL_SUBSYS_*.

Is this compromise terrible and I should give up on the enum?

enum gh_fn_type {
/* _GH_FN_* macro required for MODULE_ALIAS, otherwise __stringify() trick
 * won't work anymore */
#define _GH_FN_VCPU		1
	GH_FN_VCPU		= _GH_FN_VCPU,
#define _GH_FN_IRQFD		2
	GH_FN_IRQFD		= _GH_FN_IRQFD,
#define _GH_FN_IOEVENTFD	3
	GH_FN_IOEVENTFD		= _GH_FN_IOEVENTFD,
};

> [snip]
>
> > > +
> > > +/*
> > > + * Gunyah presently sends max 4 bytes of exit_reason.
> > > + * If that changes, this macro can be safely increased without breaking
> > > + * userspace so long as struct gh_vcpu_run < PAGE_SIZE.
> >
> > Is PAGE_SIZE allowed to be anything other than 4096 bytes?  Do you
> > expect this driver to work properly if the page size were configured
> > to be 16384 bytes?  In other words, is this a Gunyah constant, or
> > is it *really* the page size configured for Linux?
>
> Our implementations are only doing 4096 bytes. I expect the driver to 
> work properly when using 16k pages. This really is a Linux page. It's a 
> reflection of the alloc_page in gh_vcpu_bind().
>
> The exit reason is copied from hypervisor into field accessible by 
> userspace directly. Gunyah makes the exit reason size dynamic -- there's 
> no architectural limitation preventing the exit reason from being a 
> string or some lengthy data.
>
> As I was writing this response, I realized that I should be able to make 
> this a zero-length array and ensure that reason[] doesn't overflow 
> PAGE_SIZE...
>
> The comment was trying to explain that Linux itself imposes a limitation 
> on the maximum exit reason size. If we need to support longer exit 
> reason, we're OK to do so long as the total size doesn't overrun 
> PAGE_SIZE. There aren't any plans to need longer exit reasons than the 8 
> bytes mentioned today.
>
> Thanks,
> Elliot