Re: [PATCH v5 1/4] xen/arm: Implement PSCI SYSTEM_SUSPEND call for guests

[Julien Grall <julien@xxxxxxx>]

Hi Mykola,

On 02/07/2025 23:27, Mykola Kvach wrote:
> On Wed, Jul 2, 2025 at 3:28 PM Julien Grall <julien@xxxxxxx> wrote:
> > Why should we return an error? This is valid for a 64-bit domain to use
> > SMC32 convention.
> I mean — in that case, is it possible that the upper 32 bits are set to
> non-zero values without it being an explicit error from the guest?
>
> In my code, the macro used to extract 64-bit values (on 64-bit Xen, of
> course) just copies values from the Xn registers directly.
>
> According to the SMC Calling Convention specification:
> "System Software on ARM Platforms" (ARM DEN 0028A), we must use Wn
> for SMC32 parameters in AArch64.
The version A is more than 12 years old. You want to use the latest 
version. From the SMCCC DEN0028G [1] section 3.1 (Register use in 
AArch64 SMC and HVC calls):
"
The working size of the register is identified by its name:
• Xn: All 64-bits are used.
• Wn: The least significant 32-bits are used, and the most significant 
32-bits are zero. Implementations must
ignore the most significant bits.
"

So...

> AFAIK, writing to Wn zeroes the top 32 bits of Xn. So, if the guest
> is properly using 32-bit values for arguments, the upper bits will already
> be zeroed.
... while the guest should write 0 in the top 32-bit, we should not 
reject not reject non-zero values nor do nothing. Instead we should 
ignore the top bits.
Also, per the Arm Arm (ARM DDI 0487J.a) page D1-5406, it is 
implementation defined on whether the top 32-bits are zeroed when the 
previous exception context was AArch32. Xen will zero them on entry to 
avoid any surprise (see [2]), but that's only guarantee if this is a 
32-bit domain (running either on 64-bit or 32-bit Xen) as SMC can only 
be called from EL1.
As a side note, KVM is also ignoring the top 32-bits (see [3]).

Cheers,

[1] https://developer.arm.com/documentation/den0028/gbet0/?lang=en
[2] 
https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=32365f3476ac4655f2f26111cd7879912808cd77
[3] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/kvm/psci.c#n223
--
Julien Grall