Re: [PATCH v6 1/4] xen/riscv: probe the PTE A/D bits update scheme at boot

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.10.2026 13:35, Baptiste Le Duc wrote:
> On 2026-10-15 18:04 +0200, Jan Beulich wrote:
> Based on Andrew's answer to your question on GitHub [1], I see two options
> for handling a DT that doesn't advertise the scheme in effect. Please
> feel free to choose one.
> 
> 1) Trust the probe and fix up riscv_isa on a mismatch. This avoids having
> people edit their DTs, and is safe as, per Andrew, the spec has only ever
> allowed these two schemes:
> 
> /*
>  * The privileged spec allows exactly two schemes for the PTE A and D bits:
>  *  1) hardware sets them on access and on write.
>  *  2) accessing a page whose A and/or D bit is clear raises a page fault,
>  *     so that software can set them (ratified as Svade).
>  *
>  * Hence an implementation without Svade uses the hardware scheme (1), and one
>  * that faults on clear A/D bits implements Svade, whether or not it says so.
>  *
>  * Rather than trusting the DTS alone, probe the scheme in effect with a write
>  * through a PTE whose A and D bits are clear. If the DTS does not advertise
>  * it, replace it by the probed scheme. As the spec has only ever allowed
>  * these two schemes, this is safe even on implementations that predate Svade
>  * and Svadu.
>  */
> static void __init check_ad_scheme(void)
> {
>     bool has_hw_scheme = probe_hw_ad_update();
>     const char *scheme_name = has_hw_scheme ? "Svadu" : "Svade";
>     enum riscv_isa_ext_id ext = has_hw_scheme ? RISCV_ISA_EXT_svadu
>                                               : RISCV_ISA_EXT_svade;
> 
>     /* Fix DTS in case of mismatch */
>     if ( !riscv_isa_extension_available(NULL, ext) )
>     {
>         printk(XENLOG_WARNING
>                "CPU implements %s but the DTS does not advertise it\n",
>                scheme_name);
>         __clear_bit(RISCV_ISA_EXT_svade, riscv_isa);
>         __clear_bit(RISCV_ISA_EXT_svadu, riscv_isa);
>         __set_bit(ext, riscv_isa);
>     }
> 
>     printk(XENLOG_DEBUG "CPU is using %s A/D bits scheme\n", scheme_name);
> }

Yes, if the assumption is that this would be used only when neither extension
is advertised, then I agree (taking Andrew's response into account). I'm sorry,
this has been quite a bit of back and forth, but hopefully we're now all on the
same page.

Jan