Re: [PATCH v1] arm/optee: Use only least 32 bits for SMC type arg according to SMCCC

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

Hi Julien,

Julien Grall writes:

> Hi Roman,
>
> On 06/01/2021 11:26, Roman Skakun wrote:
>> This patch added additional sanity and increases an understanding for
>> getting proper value from the first argument for SMC call on aarch64
>> according to SMCC Convention.
>
> I would suggest the following commit message:
>
> "xen/arm: optee: The function identifier is always 32-bit
>
> Per the SMCCC specification (see section 3.1 in ARM DEN 0028D), the
> function identifier is only stored in the least significant
> 32-bits. The most significant 32-bits should be ignored.
>
> The function optee_handle_call() is now updated to ignore the most
> significant 32-bits.
>
> "
>
> Note that I used the version D rather than B because the latter has
> buggy wording (it seems to suggest that the least significants bits 
> should be ignored).
>
> Furthermore, I checked vsmc.c (the layer that dispatch the SMC) and it
> was already handled properly thanks to commit 7f4217cc6057 "xen/arm: 
> vsmc: The function identifier is always 32-bit".
>
>> [0] ARM DEN0028B, page 12
>> Signed-off-by: Roman Skakun <roman_skakun@xxxxxxxx>
>> ---
>>   xen/arch/arm/tee/optee.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>> diff --git a/xen/arch/arm/tee/optee.c b/xen/arch/arm/tee/optee.c
>> index ee85359742..87060b52b8 100644
>> --- a/xen/arch/arm/tee/optee.c
>> +++ b/xen/arch/arm/tee/optee.c
>> @@ -1643,7 +1643,8 @@ static bool optee_handle_call(struct cpu_user_regs 
>> *regs)
>>       if ( !ctx )
>>           return false;
>>   -    switch ( get_user_reg(regs, 0) )
>> +    /* Only least 32 bits are significant (see ARM DEN 0028B, page 12) */
>
> I would suggest:
>
> /* The function identifier is always stored in the least significant
> 32-bit (see section ARM DEN 0028D) */
>
> I can update it while committing, if both Volodymyr and you are happy
> with changes.

I'm totally fine with the proposed changes. Thank you for tidying this up.

-- 
Volodymyr Babchuk at EPAM