On 05/15/2015 09:28 AM, David Daney wrote:
On 05/14/2015 06:34 PM, Leonid Yegoshin wrote:
SEGBITS default is 40 bits or less, depending from CPU type.
This patch introduces 48bits of application virtual address (SEGBITS) support.
It is defined only for 16K and 64K pages and is optional (configurable).
Penalty - a small number of additional pages for generic (small) applications.
But for 64K pages it adds 3rd level of PTE structure, which has a little
impact during software TLB refill.
This patch is needed because MIPS I6XXX and P6XXX cores have 48 bit of
virtual address in each segment (SEGBITS).
I'm concerned that the change log doesn't convey the true reason for the
patch.
Many processors support larger VA space than is utilized by the kernel.
A choice was made to reduce the size of the VA space in order to
reduce TLB handling overhead.
If the true reason for the patch is to enable larger VA space, say that.
But is it really required by those processors you mention? I doubt it.
David Daney
Well, I was not aware about many processors capability, I can't find
this kind of note anywhere.
And I assumed that statement "If unsure, say N" and the fact that it is
configurable leads to conclusion that it is completely optional. I have
a request from management to support 48bit VA but I understand that
somebody may not like even small penalty.
In other side, this patch was required to test HW capability, GLIBC and
application compatibility (I tested on buildroot FS).
So, if you still have concern, please propose a description statement.
- Leonid.
