Re: [PATCH v6 4/5] xen/arm: report clock_frequency via sysctl physinfo, not createdomain

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Mon, Sep 28, 2026 at 02:20:00PM +0200, Julian Vetter wrote:
> The xen_arch_domainconfig.clock_frequency value is populated in
> domain_vtimer_init() during XEN_DOMCTL_createdomain from the global
> timer_dt_clock_frequency, which comes from the host's DT timer node and
> has nothing to do with the domain being created. Like now removed
> GIC_NATIVE resolution, this is a host-wide system property being
> smuggled out through a domain-creation IN struct.
> 
> Expose it instead as a new arm_clock_frequency_hz field in
> XEN_SYSCTL_physinfo, populated via arch_do_physinfo(), and mirroring how
> the GIC capability bits were already moved there.
> 
> Rather than making the field dependant on DT boot, make Xen always
> report the timer frequency, either via the DT "clock-frequency" node, or
> directly via CNTFRQ_EL0. preinit_xen_time() already computes cpu_khz for
> every boot path. The renamed timer_clock_frequency_hz now captures
> whichever of the two produced that value, in full Hz precision, instead
> of only recording the DT case. So, ACPI guests get a real value too,
> allowing to drop the special case.
> 
> The DT "clock-frequency" property exists because firmware might leave
> CNTFRQ_EL0 wrong, and since CNTFRQ_EL0 cannot be trapped the only fix is
> to replicate the correct value into the guest DT. To keep that signal, a
> new XEN_SYSCTL_PHYSCAP_ARM_TIMER_DT_FREQ capability bit records whether
> arm_clock_frequency_hz came from the DT property. Then libxl only emits
> a "clock-frequency" property into the guest timer node when that bit is
> set. A guest whose CNTFRQ_EL0 is already correct keeps an unmodified
> timer node, exactly as before.
> 
> Although the CNTFRQ_EL0 register is 64 bits wide, and some current timer
> implementations run at 1GHz, a 32bit value is sufficient to store the
> timer value, because it only mirrors the DT 'clock-frequency' property,
> which the bindings define as a single 32-bit cell.
> 
> In struct xen_sysctl_physinfo the new field just reuses the former pad
> word, so sysctl consumers are unaffected. struct xen_arch_domainconfig
> however loses clock_frequency from its middle, which shrinks the struct
> and shifts every field after it, so bump XEN_DOMCTL_INTERFACE_VERSION.
> 
> The xen_arch_domainconfig parameter passed to domain_vtimer_init() is no
> longer needed, so drop that parameter entirely. libxl now fetches the
> frequency via libxl_get_physinfo() in libxl__arch_domain_save_config()
> instead of reading it back out of the createdomain reply. The OCaml
> xen_arch_domainconfig mirror drops the field too.
> 
> Signed-off-by: Julian Vetter <julian.vetter@xxxxxxxxxx>
> Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
> ---
> Changes in v6:
> - Rename arch_clock_frequency_hz to arm_clock_frequency_hz
> - Drop now-redundant "# ARM only" comment on the libxl IDL entry
> - Rework libxl__arch_domain_save_config() to a single exit point: fetch
>   physinfo up front and use "goto out" on every error path
> - Move the gic_version IN/OUT -> IN comment update to the GIC_NATIVE
>   removal patch
> - Added Reviewed-by

Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

Thanks,


--
Anthony Perard | Vates XCP-ng Developer

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