On Mar 23, 2007, at 9:07 AM, Hollis Blanchard wrote:

On Fri, 2007-03-23 at 08:52 +0100, Christian Ehrhardt wrote:
Jimi Xenidis wrote:
...
Ensure MMCR0[FCH] for this first step:
-(ensure) set MMCR[FCH] always in xen when entering xen space. This
should prevent a domain
 messing up MMCR0[FCH]
->EXCEPTION_HEAD in exception.S set MMCR0[FCH] always

You need at least the following instructions:
    mfspr r0, SPRN_MMCR0
    ori r0, r0, 1 /* MMRC0_FCH */
    mtspr 795, SPRN_MMCR0

Unfortuantely, there is not enough room in EXCEPTION_HEAD for that and
you will get:

  exceptions.S: Assembler messages:
  exceptions.S:246: Error: attempt to .org/.space backwards? (-4)
  exceptions.S:253: Error: attempt to .org/.space backwards? (-4)
  exceptions.S:260: Error: attempt to .org/.space backwards? (-4)
  exceptions.S:267: Error: attempt to .org/.space backwards? (-4)
  exceptions.o: Bad value
  exceptions.S:626: FATAL: Can't write exceptions.o: Bad value


I suggest we start a new macro PMU_SAVE_STATE(save,scratch), which
does the above (for now, using only scratch) and sprinkling it in all
the code that EXCEPTION_HEAD branches to.

Yep, I had those already:
         mfspr r0,SPRN_MMCR0             /* ensure MMCR0[FCH]=1 */
         ori r0,r0,MMCR0_FCH
         mtspr SPRN_MMCR0, r0

Do you mean to insert a branch to PMU_SAVE_STATE from EXCEPTION_HEAD or to put a PMU_SAVE_STATE branch to every section calling EXCEPTION_HEAD ?
Can someone please point at a code piece that does a similar macro
definition and branch already.

We have 0x80 bytes for the "top half" of our exception handlers. This is dictated by hardware: there are vectors at 0x300 and 0x380, so our code must fit in that much space. If you do the math, that means we have room
for 32 instructions, including a branch to the rest of the handler
(named "ex_*_continued"). Right now, we use 30 instructions. So without doing major exception handler surgery, your three new instructions will
need to go after the branch.

Side note: I think it's ridiculous that our architecture allows the
guest to set MMCR0:FCH, since that bit controls the CPU while in
hypervisor mode. Anyways, disabling that bit a few dozen instructions
later shouldn't make a difference, and the domain never should have set
it in the first place.

Alternatively we could setup an "assist" session with screen -x
developing this part together if there is a volunteer.

I created this one blindly now as a start

     .macro PMU_SAVE_STATE save scratch
mfspr \scratch,SPRN_MMCR0 /* ensure MMCR0[FCH] =1,
not yet saving anything */
         ori \scratch,\scratch,MMCR0_FCH
         mtspr SPRN_MMCR0, \scratch
     .endm

and added the following to EXCEPTION_HEAD, but I guess it it wrong that
way ;)

     PMU_SAVE_STATE r0 r0

Right, that won't fit in EXCEPTION_HEAD (you will get the assembler
error messages Jimi pasted above).

Yeah,
So EXCEPTION_HEAD branches to a passed in label. Find all those labels and insert PMU_SAVE_STATE there.

-JX



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