Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

["H. Peter Anvin" <hpa@zytor.com>]

Jeremy Fitzhardinge wrote:
> 
> I did have some patches to do that at one point.  If you set pgd_val = 
> paravirt_nop, then the patching machinery will completely nop out the 
> call site.  The problem is that it depends on the calling convention 
> using the same regs for the first arg and return - true for 32-bit, but 
> not 64.  We could fix that with identity functions which the patcher 
> recognizes and can replace with either pure nops or inline appropriate 
> register moves.
> 

There is also the option to use assembly wrappers to avoid relying on 
the calling convention.  This is particularly so since we have sites 
where as little as a two-byte instruction gets bloated up with huge 
push/pop sequences around a tiny instruction.  Those would be better 
served with a direct call to a stub (5 bytes), which would be repatched 
to the two-byte instruction + 3 byte nop.

	-hpa