Re: [PATCH v2 07/10] arm/mm: Get rid of READ/WRITE_SYSREG32

[Julien Grall <julien@xen.org>]

On 27/04/2021 10:35, Michal Orzel wrote:
> AArch64 registers are 64bit whereas AArch32 registers
> are 32bit or 64bit. MSR/MRS are expecting 64bit values thus
> we should get rid of helpers READ/WRITE_SYSREG32
> in favour of using READ/WRITE_SYSREG.
> We should also use register_t type when reading sysregs
> which can correspond to uint64_t or uint32_t.
> Even though many AArch64 registers have upper 32bit reserved
> it does not mean that they can't be widen in the future.
> 
> Modify SCTLR_EL2 accesses to use READ/WRITE_SYSREG.
> 
> SCTLR_EL2 already has bits defined in the range [32:63].
> The ARM ARM defines them as unknown if implemented.

This is a bit ambiguous.

> By writing in head.S SCTLR_EL2_SET we are zeroing the upper
> 32bit half which is correct but referring to this sysreg
> as 32bit is a latent bug because the top 32bit was not used
> by Xen.

This seems to suggest the patch below will call SCTLR_EL2_SET whereas 
this is already existing code.

> 
> Signed-off-by: Michal Orzel <michal.orzel@arm.com>
> ---
> Changes since v1:
> -Update commit message with SCTLR_EL2 analysis
> ---
>   xen/arch/arm/mm.c    | 2 +-
>   xen/arch/arm/traps.c | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 59f8a3f15f..0e07335291 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -613,7 +613,7 @@ void __init remove_early_mappings(void)
>    */
>   static void xen_pt_enforce_wnx(void)
>   {
> -    WRITE_SYSREG32(READ_SYSREG32(SCTLR_EL2) | SCTLR_Axx_ELx_WXN, SCTLR_EL2);
> +    WRITE_SYSREG(READ_SYSREG(SCTLR_EL2) | SCTLR_Axx_ELx_WXN, SCTLR_EL2);
>       /*
>        * The TLBs may cache SCTLR_EL2.WXN. So ensure it is synchronized
>        * before flushing the TLBs.
> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> index c7acdb2087..e7384381cc 100644
> --- a/xen/arch/arm/traps.c
> +++ b/xen/arch/arm/traps.c
> @@ -915,7 +915,7 @@ static void _show_registers(const struct cpu_user_regs *regs,
>       printk(" VTTBR_EL2: %016"PRIx64"\n", ctxt->vttbr_el2);
>       printk("\n");
>   
> -    printk(" SCTLR_EL2: %08"PRIx32"\n", READ_SYSREG32(SCTLR_EL2));
> +    printk(" SCTLR_EL2: %"PRIregister"\n", READ_SYSREG(SCTLR_EL2));

Your commit title suggests that you will modify mm.c but you are also 
modifying traps.c. So how about the following commit message:

"
xen/arm: Always access SCTLR_EL2 using {READ, WRITE}_SYSREG()

The Armv8 specification describes the system register as a 64-bit value 
on AArch64 and 32-bit value on AArch32 (same as Armv7).

Unfortunately, Xen is accessing the system registers using {READ, 
WRITE}_SYSREG32() which means the top 32-bit are clobbered.

This is only a latent bug so far because Xen will not yet use the top 
32-bit.

There is also no change in behavior because arch/arm/arm64/head.S will 
initialize SCTLR_EL2 to a sane value with the top 32-bit zeroed.
"

Cheers,

-- 
Julien Grall