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Re: [PATCH] xen/arm64: Zero the top 32 bits of gp registers on entry...

Hi,

Replying in one e-mail the comments from Jan and Michal.

On 14/12/2021 10:01, Jan Beulich wrote:

On 14.12.2021 10:51, Michal Orzel wrote:

Hi Julien,

On 14.12.2021 10:33, Julien Grall wrote:



On 14/12/2021 09:17, Michal Orzel wrote:

Hi Julien, Jan


Hi,


On 08.12.2021 10:55, Julien Grall wrote:

Hi,

On 08/12/2021 07:20, Jan Beulich wrote:

On 07.12.2021 20:11, Julien Grall wrote:



On 07/12/2021 08:37, Michal Orzel wrote:

Hi Julien,


Hi,


On 06.12.2021 16:29, Julien Grall wrote:

Hi,

On 06/12/2021 14:20, Michal Orzel wrote:

to hypervisor when switching to AArch32 state.


I will change to "from AArch32 state".

According to section D1.20.2 of Arm Arm(DDI 0487A.j):
"If the general-purpose register was accessible from AArch32 state the
upper 32 bits either become zero, or hold the value that the same
architectural register held before any AArch32 execution.
The choice between these two options is IMPLEMENTATIONDEFINED"


Typo: Missing space between IMPLEMENTATION and DEFINED.


Ok.


Currently Xen does not ensure that the top 32 bits are zeroed and this
needs to be fixed.


Can you outline why this is a problem and why we need to protect? IIRC, the 
main concern is Xen may misinterpret what the guest requested but we are not 
concerned about Xen using wrong value.


I would say:
"
The reason why this is a problem is that there are places in Xen where we 
assume that top 32bits are zero for AArch32 guests.
If they are not, this can lead to misinterpretation of Xen regarding what the 
guest requested.
For example hypercalls returning an error encoded in a signed long like 
do_sched_op, do_hmv_op, do_memory_op would return -ENOSYS
if the command passed as the first argument was clobbered,
"


Fix this bug by zeroing the upper 32 bits of these registers on an
entry to hypervisor when switching to AArch32 state.

Set default value of parameter compat of macro entry to 0 (AArch64 mode
as we are on 64-bit hypervisor) to avoid checking if parameter is blank
when not passed.


Which error do you see otherwise? Is it a compilation error?


Yes, this is a compilation error. The errors appear at each line when "entry" is called 
without passing value for "compat".
So basically in all the places where entry is called with hyp=1.
When taking the current patch and removing default value for compat you will 
get:
```
entry.S:254: Error: ".endif" without ".if"
entry.S:258: Error: symbol `.if' is already defined
entry.S:258: Error: ".endif" without ".if"
entry.S:262: Error: symbol `.if' is already defined
entry.S:262: Error: ".endif" without ".if"
entry.S:266: Error: symbol `.if' is already defined
entry.S:266: Error: ".endif" without ".if"
entry.S:278: Error: symbol `.if' is already defined
entry.S:278: Error: ".endif" without ".if"
entry.S:292: Error: symbol `.if' is already defined
entry.S:292: Error: ".endif" without ".if"
entry.S:317: Error: symbol `.if' is already defined
entry.S:317: Error: ".endif" without ".if"
```


Thanks for input. I am concerned with your suggested approach (or using
.if 0\compat as suggested by Jan) because they allow the caller to not
properly specify compat when hyp=0. The risk here is we may generate the
wrong entry.

compat should need to be specified when hyp=1 as we will always run in
aarch64 mode. So could we protect this code with hyp=0?


Since my suggestion was only to avoid the need for specifying a default
for the parameter (which you didn't seem to be happy about), it would
then merely extend to

.if !0\hyp && 0\compat

Isn't it effectively the same as setting a default value?

The reason we seem to get away is because other part of the macro (e.g. 
entry_guest) will need compat to be valid.

But that seems pretty fragile to me. So I would prefer if the new code it added 
within a macro that is only called when hyp==0.


So you would like to have a macro that is called if hyp=0 (which means compat 
had to be passed) and inside this macro additional check if compat is 1?


Yes. This is the only way I could think to avoid making 'compat'optional.


Cheers,



or something along those lines.

Jan




So when it comes to zeroing the top 32bits by pushing zero to higher halves of 
stack slots I would do in a loop:
stp wzr, wzr, [sp #8 * 0]
stp wzr, wzr, [sp #8 * 1]
...


I don't think you can use stp here because this would store two 32-bit values 
consecutively. Instead, you would need to use ldr to store one 32-bit value at 
the time.


I hope you meant str and not ldr.


Yes. I am not sure why I wrote ldr.


So a loop would look like that:
str wzr, [sp, #8 * 1]
str wzr, [sp, #8 * 3]
...


Why "a loop" and why #8 (I'd have expected #4)?

There's another oddity which I'm noticing only now, but which also
may look odd to me only because I lack sufficient Arm details: On
x86, it would not be advisable to store anything below the stack
pointer (like is done here when storing x0 and x1 early), unless
it's absolutely certain that no further interruptions could clobber
that part of the stack.
We are entering the hypervisor with both Interrupts and SErrors masked. They will only be unmasked after the guest registers have been saved on the stack.
You may still receive a Data Abort before the macro 'entry' has completed. But this is going to result to an hypervisor crash because they are not meant to happen in those paths. 

So I believe, we are safe to modify sp before.

Cheers,

--
Julien Grall

 

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