Re: [PATCH v3] xen/arm64: Zero the top 32 bits of gp registers on entry...

[Michal Orzel <michal.orzel@xxxxxxx>]

Hi Julien,

On 17.12.2021 11:01, Julien Grall wrote:
> Hi,
> 
> On 17/12/2021 07:21, Michal Orzel wrote:
>> to hypervisor when switching 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 IMPLEMENTATION DEFINED"
>>
>> Currently Xen does not ensure that the top 32 bits are zeroed and this
>> needs to be fixed. The reason why 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.
>>
>> Create a macro clobber_gp_top_halves to clobber top 32 bits of gp
>> registers when hyp == 0 (guest mode) and compat == 1 (AArch32 mode).
>> Add a compile time check to ensure that save_x0_x1 == 1 if
>> compat == 1.
>>
>> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
>> ---
>> Changes since v2:
>> -add clobbering of w30
>> Changes since v1:
>> -put new code into macro
>> -add compile time check for save_x0_x1
>> ---
>>   xen/arch/arm/arm64/entry.S | 29 +++++++++++++++++++++++++++++
>>   1 file changed, 29 insertions(+)
>>
>> diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
>> index fc3811ad0a..e351ef8639 100644
>> --- a/xen/arch/arm/arm64/entry.S
>> +++ b/xen/arch/arm/arm64/entry.S
>> @@ -102,6 +102,30 @@
>>           .endif
>>             .endm
>> +
>> +/*
>> + * Clobber top 32 bits of gp registers when switching from AArch32
>> + */
>> +        .macro clobber_gp_top_halves, compat, save_x0_x1
>> +
>> +        .if \compat == 1      /* AArch32 mode */
>> +
>> +        /*
>> +         * save_x0_x1 is equal to 0 only for guest_sync (compat == 0).
>> +         * Add a compile time check to avoid violating this rule.
>> +         */
> 
> We may want in the future to allow save_x0_x1 == 1 with compat == 1 if we 
> need to create fastpath like we did when entering AArch64.
> 
> So I would reword the comment to make clear this is an implementation 
> decision. How about:
> 
> "At the moment, no-one is using save_x0_x1 == 0 with compat == 1. So the code 
> is not handling it to simplify the implementation."
> 
> If you are happy with the new comment, I can update it on commit:
> 
Please do. The comment looks ok.

> Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
> 
> Cheers,
> 

Cheers,
Michal