Re: Need guidance to support reading GICR_TYPER (64 bit register) on Aarch32_v8r

[Julien Grall <julien@xxxxxxx>]

Hi Ayan,

On 13/10/2022 15:30, Ayan Kumar Halder wrote:
> On 13/10/2022 14:20, Julien Grall wrote:
> > On 13/10/2022 12:47, Ayan Kumar Halder wrote:
> > > Hi Arm mantainers/Folks,
> > Hello,
> Hi Julien,
>
> Appreciate your help as always. :)
>
> > > Please refer to the discussion 
> > > https://github.com/zephyrproject-rtos/zephyr/pull/51163 .
> > > We intend to run Zephyr as a domU guest on Xen on Aarch32_v8R fixed 
> > > virtual platform.
> > > Zephyr is trying to read GICR_TYPER which is a 64 bit register using 
> > > ldrd instruction.
> > > As GICR is emulated by Xen, so this instruction gets trapped with HSR 
> > > = 0x9200000c.
> > > As ISV is 0, so Xen cannot emulate this instruction.
> > >
> > > The proposed solution is to use two sys_read32() on GICR_TYPER to 
> > > return the lower and upper 32 bits.
> > > With this, HSR = 0x9383 000c, ISV=1 so ISS is valid.
> > >
> > > Now, for Xen to emulate this read, I have proposed the modifications 
> > > (in my last comment).
> > I am confused. Looking at the emulation of GICR_TYPER in Xen 
> > (arch/arm/vgic-v3.c), the code should already be able to handle 32-bit 
> > access.
> When I compile Xen for arm32, vreg_reg64_extract() definitions do not 
> exist.
Ah. You are building Xen for arm32... This is unsupported by Xen at the 
moment.
> The reason being 
> https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/arm/include/asm/vreg.h;h=f26a70d024e2d1f933ea66793ea6e42f81c7a8cf;hb=9029bc265cdf2bd63376dde9fdd91db4ce9c0586#l189 , BITS_PER_LONG is 32 (Refer xen/arch/arm/include/asm/config.h).
Right, so that's not an architecture issue (like 'ldrd') but just Xen 
not providing the helper.
As I wrote above, Xen doesn't yet support GICv3 on arm32. I expect that 
implementing vreg_reg64_extract() will be one of the step.
> .Thus, the guest need to invoke sys_read32() twice (GICR_TYPER and 
> GICR_TYPER+4).
I don't understand how you came to this conclusion with what you wrote. 
If we had implemented vreg_reg64_extract(), then Zephyr would still need 
to issue two 32-bit read because Xen doesn't emulate 'ldrd'.
Cheers,

--
Julien Grall