linux-kernel - Re: [PATCH v3 6/7] xen: allow mapping ACPI data using a different physical address

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Message-ID: <6bed58f8-016f-4390-be4c-128eebff6545@suse.com>
Date: Tue, 10 Sep 2024 14:34:58 +0200
From: Jan Beulich <jbeulich@...e.com>
To: Juergen Gross <jgross@...e.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, "Rafael J. Wysocki" <rafael@...nel.org>,
 Len Brown <lenb@...nel.org>, Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
 x86@...nel.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v3 6/7] xen: allow mapping ACPI data using a different
 physical address

On 10.09.2024 12:39, Juergen Gross wrote:
> When running as a Xen PV dom0 the system needs to map ACPI data of the
> host using host physical addresses, while those addresses can conflict
> with the guest physical addresses of the loaded linux kernel. The same
> problem might apply in case a PV guest is configured to use the host
> memory map.
> 
> This conflict can be solved by mapping the ACPI data to a different
> guest physical address, but mapping the data via acpi_os_ioremap()
> must still be possible using the host physical address, as this
> address might be generated by AML when referencing some of the ACPI
> data.
> 
> When configured to support running as a Xen PV domain, have an
> implementation of acpi_os_ioremap() being aware of the possibility to
> need above mentioned translation of a host physical address to the
> guest physical address.
> 
> This modification requires to fix some #include of asm/acpi.h in x86
> code to use linux/acpi.h instead.
> 
> Signed-off-by: Juergen Gross <jgross@...e.com>

Reviewed-by: Jan Beulich <jbeulich@...e.com>
with a request to comment a tiny bit more:

> @@ -836,6 +837,33 @@ void __init xen_do_remap_nonram(void)
>  	pr_info("Remapped %u non-RAM page(s)\n", remapped);
>  }
>  
> +#ifdef CONFIG_ACPI
> +/*
> + * Xen variant of acpi_os_ioremap() taking potentially remapped non-RAM
> + * regions into acount.

(Nit: account)

> + * Any attempt to map an area crossing a remap boundary will produce a
> + * WARN() splat.
> + */
> +static void __iomem *xen_acpi_os_ioremap(acpi_physical_address phys,
> +					 acpi_size size)
> +{
> +	unsigned int i;
> +	const struct nonram_remap *remap = xen_nonram_remap;
> +
> +	for (i = 0; i < nr_nonram_remap; i++) {
> +		if (phys + size > remap->maddr &&
> +		    phys < remap->maddr + remap->size) {
> +			WARN_ON(phys < remap->maddr ||
> +				phys + size > remap->maddr + remap->size);
> +			phys = remap->paddr + phys - remap->maddr;

This might be slightly easier / more logical to read as

			phys += remap->paddr - remap->maddr;

Also because of "phys" not consistently expressing a physical address
(when you need convert it, the incoming value is a machine address) a
comment may help here. In fact at the first glance (and despite having
seen the code before) I thought the translation was done the wrong way
round, simply because of the name of the variable.

Jan