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

On 06/01/2021 23:22, Volodymyr Babchuk wrote:

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.

Thanks! I have updated the patch and committed it.


Julien Grall



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