Jimi Xenidis wrote:

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

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).

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


I need a register to clobber with my "ori". (H_)EXCEPTION_SAVE_STATE clobbers r0 anyway and it resides in all continue labels that are passed to EXCEPTION_HEAD. So PMU_SAVE_STATE can follow in its wake and clobber r0 too like EXCEPTION_SAVE_STATE.

branches to EXCEPTION_HEAD:
xenppc-unstable_step1/xen# cat arch/powerpc/powerpc64/exceptions.S | grep "EXCEPTION_HEAD" | sort | uniq
           EXCEPTION_HEAD r13 ex_dec_continued
           EXCEPTION_HEAD r13 ex_external_continued
           EXCEPTION_HEAD r13 ex_hcall_continued
           EXCEPTION_HEAD r13 ex_hdec_continued
           EXCEPTION_HEAD r13 ex_machcheck_continued
           EXCEPTION_HEAD r13 ex_program_continued

One of those calls as example "diff -Naur":
@@ -384,6 +391,7 @@
    mr r14, r0
+    PMU_SAVE_STATE r0,r0
    mr r4, r14
    LOADADDR r12, program_exception
    mr r3, r1                           /* pass pointer to cpu_user_regs */

Please correct me If my assumption is wrong that I'm allowed to clobber r0 there.


Grüsse / regards, Christian Ehrhardt

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