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Date:   Mon, 21 Nov 2022 15:10:36 +0100
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Subject: Re: [PATCH 2/3] acpi/processor: sanitize _PDC buffer bits when
 running as Xen dom0
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To:     Roger Pau Monne <roger.pau@citrix.com>
Cc:     xen-devel@lists.xenproject.org, jgross@suse.com,
        stable@vger.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <lenb@kernel.org>, linux-acpi@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20221121102113.41893-1-roger.pau@citrix.com>
 <20221121102113.41893-3-roger.pau@citrix.com>
From:   Jan Beulich <jbeulich@suse.com>
In-Reply-To: <20221121102113.41893-3-roger.pau@citrix.com>
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On 21.11.2022 11:21, Roger Pau Monne wrote:
> --- a/drivers/acpi/processor_pdc.c
> +++ b/drivers/acpi/processor_pdc.c
> @@ -137,6 +137,14 @@ acpi_processor_eval_pdc(acpi_handle handle, struct acpi_object_list *pdc_in)
>  		buffer[2] &= ~(ACPI_PDC_C_C2C3_FFH | ACPI_PDC_C_C1_FFH);
>  
>  	}
> +	if (xen_initial_domain())
> +		/*
> +		 * When Linux is running as Xen dom0 it's the hypervisor the
> +		 * entity in charge of the processor power management, and so
> +		 * Xen needs to check the OS capabilities reported in the _PDC
> +		 * buffer matches what the hypervisor driver supports.
> +		 */
> +		xen_sanitize_pdc((uint32_t *)pdc_in->pointer->buffer.pointer);
>  	status = acpi_evaluate_object(handle, "_PDC", pdc_in, NULL);

Again looking at our old XenoLinux forward port we had this inside the
earlier if(), as an _alternative_ to the &= (I don't think it's valid
to apply both the kernel's and Xen's adjustments). That would also let
you use "buffer" rather than re-calculating it via yet another (risky
from an abstract pov) cast.

It was the very nature of requiring Xen-specific conditionals which I
understand was the reason why so far no attempt was made to get this
(incl the corresponding logic for patch 1) into any upstream kernel.

Jan