* Re: [XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator
2022-12-02 12:56 ` Andrew Cooper
@ 2022-11-20 17:01 ` Bobby Eshleman
2022-12-03 20:10 ` Ayan Kumar Halder
0 siblings, 1 reply; 6+ messages in thread
From: Bobby Eshleman @ 2022-11-20 17:01 UTC (permalink / raw)
To: Andrew Cooper
Cc: Ayan Kumar Halder, xen-devel, sstabellini, stefanos, julien,
George Dunlap, jbeulich, alistair.francis, connojdavis, wl
On Fri, Dec 02, 2022 at 12:56:06PM +0000, Andrew Cooper wrote:
> On 02/12/2022 12:35, Ayan Kumar Halder wrote:
> > Currently this change will not have any impact on the existing architectures.
> > The following table illustrates PADDR_BITS vs BITS_PER_LONG of different archs
> >
> > ------------------------------------------------
> > | Arch | PADDR_BITS | BITS_PER_LONG |
> > ------------------------------------------------
> > | Arm_64 | 48 | 64 |
> > | Arm_32 | 40 | 32 |
> > | RISCV_64 | Don't know | 64 |
>
> Just FYI, I think the answer here is 56 for RISC-V.
>
> ~Andrew
>
Andrew got it: 56.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator
@ 2022-12-02 12:35 Ayan Kumar Halder
2022-12-02 12:56 ` Andrew Cooper
2022-12-02 13:43 ` Jan Beulich
0 siblings, 2 replies; 6+ messages in thread
From: Ayan Kumar Halder @ 2022-12-02 12:35 UTC (permalink / raw)
To: xen-devel
Cc: sstabellini, stefanos, julien, andrew.cooper3, george.dunlap,
jbeulich, bobbyeshleman, alistair.francis, connojdavis, wl,
Ayan Kumar Halder
We want to ensure that "xenheap_bits - PAGE_SHIFT" is strictly less than
the number of bits to represent unsigned long as it is used a rhs operand
to shift mfn.
It is also important to note that the "xenheap_bits - PAGE_SHIFT" is never
used to shift an address (rather than a frame number), and going forward
then also shouldn't be (perhaps unless further precautions are taken).
Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
---
Currently this change will not have any impact on the existing architectures.
The following table illustrates PADDR_BITS vs BITS_PER_LONG of different archs
------------------------------------------------
| Arch | PADDR_BITS | BITS_PER_LONG |
------------------------------------------------
| Arm_64 | 48 | 64 |
| Arm_32 | 40 | 32 |
| RISCV_64 | Don't know | 64 |
| x86 | 52 | 64 |
-------------------------------------------------
However, this will change when we introduce a platform (For eg Cortex-R52) which
supports 32 bit physical address and BITS_PER_LONG. This platform does not follow
the same code path as Arm_32.
Thus, I have introduced this change as I don't see it causing a regression on
any of the supported platforms.
Changes from v1:-
1. Changed the check from "(PADDR_BITS > BITS_PER_LONG)" to "((PADDR_BITS - PAGE_SHIFT) >= BITS_PER_LONG)"
2. Updated the commit message to explain the reason for this.
v2 :-
1. Updated the commit message.
xen/common/page_alloc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 62afb07bc6..c5b8c7444f 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -2245,7 +2245,7 @@ void __init xenheap_max_mfn(unsigned long mfn)
{
ASSERT(!first_node_initialised);
ASSERT(!xenheap_bits);
- BUILD_BUG_ON(PADDR_BITS >= BITS_PER_LONG);
+ BUILD_BUG_ON((PADDR_BITS - PAGE_SHIFT) >= BITS_PER_LONG);
xenheap_bits = min(flsl(mfn + 1) - 1 + PAGE_SHIFT, PADDR_BITS);
printk(XENLOG_INFO "Xen heap: %u bits\n", xenheap_bits);
}
--
2.17.1
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator
2022-12-02 12:35 [XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator Ayan Kumar Halder
@ 2022-12-02 12:56 ` Andrew Cooper
2022-11-20 17:01 ` Bobby Eshleman
2022-12-02 13:43 ` Jan Beulich
1 sibling, 1 reply; 6+ messages in thread
From: Andrew Cooper @ 2022-12-02 12:56 UTC (permalink / raw)
To: Ayan Kumar Halder, xen-devel
Cc: sstabellini, stefanos, julien, George Dunlap, jbeulich,
bobbyeshleman, alistair.francis, connojdavis, wl
On 02/12/2022 12:35, Ayan Kumar Halder wrote:
> Currently this change will not have any impact on the existing architectures.
> The following table illustrates PADDR_BITS vs BITS_PER_LONG of different archs
>
> ------------------------------------------------
> | Arch | PADDR_BITS | BITS_PER_LONG |
> ------------------------------------------------
> | Arm_64 | 48 | 64 |
> | Arm_32 | 40 | 32 |
> | RISCV_64 | Don't know | 64 |
Just FYI, I think the answer here is 56 for RISC-V.
~Andrew
> | x86 | 52 | 64 |
> -------------------------------------------------
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator
2022-12-02 12:35 [XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator Ayan Kumar Halder
2022-12-02 12:56 ` Andrew Cooper
@ 2022-12-02 13:43 ` Jan Beulich
2022-12-02 14:00 ` Julien Grall
1 sibling, 1 reply; 6+ messages in thread
From: Jan Beulich @ 2022-12-02 13:43 UTC (permalink / raw)
To: Ayan Kumar Halder
Cc: sstabellini, stefanos, julien, andrew.cooper3, george.dunlap,
bobbyeshleman, alistair.francis, connojdavis, wl, xen-devel
On 02.12.2022 13:35, Ayan Kumar Halder wrote:
> We want to ensure that "xenheap_bits - PAGE_SHIFT" is strictly less than
> the number of bits to represent unsigned long as it is used a rhs operand
> to shift mfn.
> It is also important to note that the "xenheap_bits - PAGE_SHIFT" is never
> used to shift an address (rather than a frame number), and going forward
> then also shouldn't be (perhaps unless further precautions are taken).
Hmm, now you've lost why you are making the change in the first place:
The way things are before the patch is quite fine for the described
purpose. (This then also extends to the title. I should have noticed
this in v2 already, but didn't because I still had the v1 title in mind.)
Furthermore in the 2nd paragraph instead of 'the "xenheap_bits -
PAGE_SHIFT"' you mean '"xenheap_bits" alone'.
Jan
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator
2022-12-02 13:43 ` Jan Beulich
@ 2022-12-02 14:00 ` Julien Grall
0 siblings, 0 replies; 6+ messages in thread
From: Julien Grall @ 2022-12-02 14:00 UTC (permalink / raw)
To: Jan Beulich, Ayan Kumar Halder
Cc: sstabellini, stefanos, andrew.cooper3, george.dunlap,
bobbyeshleman, alistair.francis, connojdavis, wl, xen-devel
Hi,
On 02/12/2022 13:43, Jan Beulich wrote:
> On 02.12.2022 13:35, Ayan Kumar Halder wrote:
>> We want to ensure that "xenheap_bits - PAGE_SHIFT" is strictly less than
>> the number of bits to represent unsigned long as it is used a rhs operand
>> to shift mfn.
>> It is also important to note that the "xenheap_bits - PAGE_SHIFT" is never
>> used to shift an address (rather than a frame number), and going forward
>> then also shouldn't be (perhaps unless further precautions are taken).
>
> Hmm, now you've lost why you are making the change in the first place:
> The way things are before the patch is quite fine for the described
> purpose. (This then also extends to the title. I should have noticed
> this in v2 already, but didn't because I still had the v1 title in mind.)
>
> Furthermore in the 2nd paragraph instead of 'the "xenheap_bits -
> PAGE_SHIFT"' you mean '"xenheap_bits" alone'.
Let me propose a commit message:
"
xen/page_alloc: Relax the BUILD_BUG_ON() in xenheap_max_mfn()
In the near future, we are considering to use a common xen/domain heap
for Arm 32-bit V8-R. In this setup, both PADDR_BITS and BITS_PER_LONG
will be 32. Therefore, the condition PADDR_BITS >= BITS_PER_LONG will
become true.
Looking at the commit that introduce the BUILD_BUG_ON (88e3ed61642b
"x86/NUMA: make init_node_heap() respect Xen heap limit") and the
current use, it seems this check was mainly to ensure that the shift of
xenheap_bits is not going to be undefined (>> N for a N-bit type is
undefined).
So far, all the shifts are using "xenheap_bits - PAGE_SHIFT". Therefore,
the existing code should work for 32-bit architecture as long as the
result of the substraction is below 32.
Therefore relax the BUILD_BUG_ON() to check that (PADDR_BITS -
PAGE_SHIFT) is not equal of above BITS_PER_LONG.
Note that we would need further precaution if we ended up to use
'xenheap_bits' alone in shifts.
"
If you end up to take my commit message, then please add my signed-off-by.
Cheers,
--
Julien Grall
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator
2022-11-20 17:01 ` Bobby Eshleman
@ 2022-12-03 20:10 ` Ayan Kumar Halder
0 siblings, 0 replies; 6+ messages in thread
From: Ayan Kumar Halder @ 2022-12-03 20:10 UTC (permalink / raw)
To: Bobby Eshleman, Andrew Cooper
Cc: Ayan Kumar Halder, xen-devel, sstabellini, stefanos, julien,
George Dunlap, jbeulich, alistair.francis, connojdavis, wl
Hi Bobby,
On 20/11/2022 17:01, Bobby Eshleman wrote:
> On Fri, Dec 02, 2022 at 12:56:06PM +0000, Andrew Cooper wrote:
>> On 02/12/2022 12:35, Ayan Kumar Halder wrote:
>>> Currently this change will not have any impact on the existing architectures.
>>> The following table illustrates PADDR_BITS vs BITS_PER_LONG of different archs
>>>
>>> ------------------------------------------------
>>> | Arch | PADDR_BITS | BITS_PER_LONG |
>>> ------------------------------------------------
>>> | Arm_64 | 48 | 64 |
>>> | Arm_32 | 40 | 32 |
>>> | RISCV_64 | Don't know | 64 |
>> Just FYI, I think the answer here is 56 for RISC-V.
>>
>> ~Andrew
>>
> Andrew got it: 56.
Thanks Andrew, Bobby.
Bobby, I couldn't see PADDR_BITS defined anywhere. Did I miss something ?
If not, will you define PADDR_BITS somewhere in xen/arch/riscv/* ?
- Ayan
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2022-12-03 20:11 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2022-12-02 12:35 [XEN v3] Xen: Ensure "xenheap_bits - PAGE_SHIFT" can be used as a rhs operand of shift operator Ayan Kumar Halder
2022-12-02 12:56 ` Andrew Cooper
2022-11-20 17:01 ` Bobby Eshleman
2022-12-03 20:10 ` Ayan Kumar Halder
2022-12-02 13:43 ` Jan Beulich
2022-12-02 14:00 ` Julien Grall
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