Re: [PATCH v2 1/2] x86/cpufeature: Add facility to match microcode revisions

[Thomas Gleixner <tglx@linutronix.de>]

Andi,

On Sat, 20 Oct 2018, Andi Kleen wrote:
> On Sat, Oct 20, 2018 at 10:19:37AM +0200, Thomas Gleixner wrote:
> > On Fri, 19 Oct 2018, Andi Kleen wrote:
> > There is no point to return the pointer because it's not a compound
> > structure. If you want to provide the possibility to use the index then
> > return the index and an error code if it does not match.
> 
> It will be useful with the driver_data pointer, which you short sightedly
> forced me to remove, and likely will need to be readded at some point
> anyways if this gets more widely used.

It's good and established practice not to add functionality on a 'might be
used' basis. If you'd provide at least one or two patches which demonstrate
how that is useful then that would be convincing.

>  At least with the pointer not all callers will need to be changed then.

It doesn't need to be changed at all, when done correctly.

So lets walk through that again:

1) x86_match_microcode() is a misnomer because it's not as the name
   suggests a match function. It compares whether the micro code revision
   is greater or equal the minimal required micro code revision for the
   current CPU.
   
2) None of the existing implementations needs a pointer return value,
   neither does your use case at hand.

3) If this should be extended to a generic cpu id matching facility, then
   it can be very well designed so. See below.

Step 1:

struct x86_cpu_check {
	u8	vendor;
	u8	family;
	u8	model;
	u8	stepping;
};

struct x86_cpu_check *x86_match_cpu(struct x86_cpu_check *table)
{
	// Find matching vendor, family, model, stepping entry
	... {
		return entry;
	}
	return NULL;
}

Genuine CPU match function, which can be extended by extending the data
structure.

Step 2:

struct x86_cpu_check {
	u8	vendor;
	u8	family;
	u8	model;
	u8	stepping;
	u32	microcode_rev;
};

bool x86_cpu_has_min_microcode(struct x86_cpu_check *table)
{
	struct x86_cpu_check *res = x86_match_cpu(table);

	if (!res)
		return false;
	return res->microcode_revision >= boot_cpu_data.microcode;
}

Step 3:

struct x86_cpu_check {
	u8	vendor;
	u8	family;
	u8	model;
	u8	stepping;
	union {
		u32	microcode_rev;
		void	*driver_data;
	}
};

Can be used with x86_match_cpu() for all non microcode based matching.

So if you really need something which checks the microcode and provides the
pointer, then it's easy enough to do:

Step 4:

struct x86_cpu_check {
	u8	vendor;
	u8	family;
	u8	model;
	u8	stepping;
	u32	microcode_rev;
	void	*driver_data;
};

struct x86_cpu_check *x86_check_min_microcode(struct x86_cpu_check *table)
{
	struct x86_cpu_check *res = x86_match_cpu(table);

	if (!res || res->microcode_rev < boot_cpu_data.microcode)
		return NULL;
	return res;
}

static inline bool x86_cpu_has_min_microcode(struct x86_cpu_check *table)
{
	return !!x86_check_min_microcode(table);
}

None of these steps requires to change a call site or a table.

But probably I'm too short sighted and missing something crucial. Looking
forward for enlightment.

> Also it's symmetric with how the PCI and USB and the existing x86 match
> discovery interfaces work.

And the point is? That we need to keep everything as we've done it 20 years
ago?

> > > > VENDOR_INTEL = 0, so this check is obscure to begin with. Either you chose
> > > > a explicit condition to put at the end of the table, e.g. vendor = U8_MAX
> > > > or you hand in the array size to the function.
> > > 
> > > That would both be awkward. It's the same as match_cpu, and 0 terminators
> > > are standard practice in practical all similar code. I removed
> > > the or with the family.
> > 
> > That's debatable because it's more easy to miss the terminator than getting
> > the ARRAY_SIZE() argument wrong. But it doesn't matter much.
> 
> Ok then please apply it. 

Sure, once this argument is settled and all review comments are addressed.

Thanks,

	tglx