Re: [RFC PATCH] x86/ACPI: optimise C3 entry on AMD CPUs

[Jason Andryuk <jason.andryuk@xxxxxxx>]

On 2026-09-30 14:13, Olivier Lambert wrote:
> All Zen or newer CPU which support C3 shares cache. Its not necessary to
> flush the caches in software before entering C3. This will cause drop in
> performance for the cores which share some caches. ARB_DIS is not used
> with current AMD C state implementation. So set related flags correctly.
> Signed-off-by: Deepak Sharma <deepak.sharma@xxxxxxx>
> Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Origin: https://git.kernel.org/torvalds/p/a8fb40966f19 
> [https://git.kernel.org/torvalds/p/a8fb40966f19]
> [Port to Xen. Include Hygon.]
> Assisted-by: Claude-Code:claude-opus-5-5
> Signed-off-by: Olivier Lambert <olivier.lambert@xxxxxxxxxx>
Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>

but...

> ---
> Found on XCP-ng 8.3 (Xen 4.17.6) on a Ryzen 5 7600 (Zen 4, family 0x19)
> whose _CST lists C1 (HLT) plus C2 and C3 as SYSIO states (C3 exit
> latency 350us). With bm_check == 0, every C3 entry executes WBINVD.
> Measured under a VM-to-VM network load, in 25s windows:
> - ~140 WBINVD/s, 65-69us each on average, 300us at worst.
> - Spinning on each CPU in turn with IRQs off and reading the TSC shows
> gaps of up to 222us; none with this change. The SMI count (PMC event
> 0x2b, validated by counting cycles on the same counter) stayed at 0,
> and NMIs didn't line up with the gaps.
> - credit1's do_schedule() occasionally took up to 262us, with IRQs
> off; at most 4.4us with this change.
> - xenpm set-max-cstate 1 removes the stalls too, which points at C3
> entry.
> This change was booted and measured as a backport on 4.17.6 only. On
> staging it is build-tested. It also covers Hygon, as Linux does today
> through X86_FEATURE_ZEN; the original Linux commit was AMD-only.
> This will conflict trivially with Alejandro's pending "x86/acpi: Migrate
> vendor checks to cpu_vendor()"; happy to rebase on top of it.
> Possibly a backport candidate.
> xen/arch/x86/acpi/cpu_idle.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
> diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
> index 56b22a2..b2bca4d 100644
> --- a/xen/arch/x86/acpi/cpu_idle.c
> +++ b/xen/arch/x86/acpi/cpu_idle.c
> @@ -1065,6 +1065,24 @@ static void acpi_processor_power_init_bm_check(struct 
> acpi_processor_flags *flag
> */
> if ( c->vendor == X86_VENDOR_INTEL )
> flags->bm_control = 0;
> +
> + if ( (c->vendor & (X86_VENDOR_AMD | X86_VENDOR_HYGON)) &&
... you've lost all indenting.

Regards,
Jason