Re: [PATCH 17/21] x86/fpu: Use proper mask to replace full instruction mask

From: Dave Hansen <dave.hansen@intel.com>
To: "Liang, Kan" <kan.liang@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>
Cc: mingo@redhat.com, acme@kernel.org, tglx@linutronix.de,
	bp@alien8.de, x86@kernel.org, linux-kernel@vger.kernel.org,
	mark.rutland@arm.com, alexander.shishkin@linux.intel.com,
	jolsa@redhat.com, namhyung@kernel.org, yu-cheng.yu@intel.com,
	bigeasy@linutronix.de, gorcunov@gmail.com, hpa@zytor.com,
	alexey.budankov@linux.intel.com, eranian@google.com,
	ak@linux.intel.com, like.xu@linux.intel.com,
	yao.jin@linux.intel.com
Subject: Re: [PATCH 17/21] x86/fpu: Use proper mask to replace full instruction mask
Date: Mon, 22 Jun 2020 11:05:32 -0700	[thread overview]
Message-ID: <5223f714-87eb-947e-e65c-886431cc7655@intel.com> (raw)
In-Reply-To: <56653932-4c11-60f9-1541-a19ea307c0a9@linux.intel.com>

On 6/22/20 10:47 AM, Liang, Kan wrote:
>> I'm wondering if we should just take these copy_*regs_to_*() functions
>> and uninline them.  Yeah, they are basically wrapping one instruction,
>> but it might literally be the most heavyweight instruction in the
>> whole ISA.
> 
> Thanks for the suggestions, but I'm not sure if I follow these methods.
> 
> I don't think simply removing the "inline" key word for the
> copy_xregs_to_kernel() functions would help here.
> Do you mean exporting the copy_*regs_to_*()?

The thing that worries me here is exporting "internal" FPU state like
xfeatures_mask_all.  I'm much happier exporting a function with a much
more defined purpose.

So, yes, I'm suggesting exporting the functions, *not* the data structures.

>> Or, maybe just make an out-of-line version for KVM to call?
> 
> I think the out-of-line version for KVM still needs the
> xfeatures_mask_all. Because the size of vcpu's XSAVE buffer
> (&vcpu->arch.guest_fpu) is the same as other kernel XSAVE buffers, such
> as task->fpu. The xfeatures_mask_all is required for KVM to filter out
> the dynamic supervisor feature as well. I think even if we make an
> out-of-line version for KVM, we still have to export the
> xfeatures_mask_all for KVM.

No.

You do this in a .h file:

extern void notinline_copy_xregs_to_kernel(struct xregs_state *xstate);

And then this in a .c file:

void notinline_copy_xregs_to_kernel(struct xregs_state *xstate)
{
	copy_xregs_to_kernel(xstate);
}
EXPORT_SYMBOL_GPL(notinline_copy_xregs_to_kernel);

KVM now calls notinline_copy_xregs_to_kernel() (not what it should
really be called).  It does *not* need 'xfeatures_mask_all' exported in
this case.  That preserves the inlining for core kernel users.

It's not the prettiest situation, but it is straightforward.