Re: [PATCH v3 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state

On Wed, Mar 22, 2017 at 1:49 PM, H. Peter Anvin [off-list ref] wrote:

On 03/22/17 13:41, Thomas Garnier wrote:

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with the change below for additional feedback.

Can you specify what that means?

If I set inline by default, the compiler chose not to inline it on
x86. If I force inline the size impact was actually bigger (without
the architecture specific code).

That's utterly bizarre.  Something strange is going on there.  I suspect
the right thing to do is to out-of-line the error case only, but even
that seems strange.  It should be something like four instructions inline.

The compiler seemed to often inline other functions called by the
syscall handlers. I assume the growth was due to changes in code
optimization because the function is much larger at the end.

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On x86, where there is only one caller of this, it really seems like it
ought to reduce the overhead to almost zero (since it most likely is
hidden in the pipeline.)

I would like to suggest defining it inline if
CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE is set; I really don't
care about an architecture which doesn't have it.

But if there is only one caller, does the compiler is not suppose to
inline the function based on options?

If it is marked static in the same file, yes, but you have it in a
different file from what I can tell.

If we do global optimization, it should. Having it as a static inline
make it easier on all types of builds.

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The assembly will call it too, so I would need an inline and a
non-inline based on the caller.

Where?  I don't see that anywhere, at least for x86.

After the latest changes on x86, yes. On arm/arm64, we call it with
the CHECK_DATA_CORRUPTION config.

        -hpa

-- 
Thomas