linux-kernel - Re: [PATCH 1/1] x86/cpu/topology: Don't limit CPUs to 1 for Xen PV guests due to disabled APIC

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250407132445.6732-1-arkamar@atlas.cz>
Date: Mon,  7 Apr 2025 15:24:26 +0200
From: Petr Vaněk <arkamar@...as.cz>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>,
	arkamar@...as.cz,
	Juergen Gross <jgross@...e.com>,
	linux-kernel@...r.kernel.org,
	stable@...r.kernel.org,
	x86@...nel.org,
	xen-devel@...ts.xenproject.org,
	linux-acpi@...r.kernel.org
Subject: Re: [PATCH 1/1] x86/cpu/topology: Don't limit CPUs to 1 for Xen PV guests due to disabled APIC

On Sun, Apr 06, 2025 at 12:08:29PM +0200, Thomas Gleixner wrote:
> On Sat, Apr 05 2025 at 20:16, Petr Vaněk wrote:
> 
> > Xen PV guests in DomU have APIC disabled by design, which causes
> > topology_apply_cmdline_limits_early() to limit the number of possible
> > CPUs to 1, regardless of the configured number of vCPUs.
> 
> PV guests have a APIC emulation and there is no code which actually
> disables the APIC by design unconditionally. There is one way though,
> which disables the APIC indirectly.

It seems I have got a bit lost in APIC/ACPI abbreviations. Sorry.

> xen_arch_setup() disables ACPI, which in turn causes acpi_mps_check() to
> return 1, which disables the APIC. This only happens when the kernel
> configuration has:
> 
>      CONFIG_X86_MPPARSE=n
>      CONFIG_ACPI=y
> 
> If you enable MPPARSE the problem goes away, no?

Yes, it goes away.

> > +	/* 'maxcpus=0' 'nosmp' 'nolapic'
> > +	 *
> > +	 * The apic_is_disabled check is ignored for Xen PV domains because Xen
> > +	 * disables ACPI in unprivileged PV DomU guests, which would otherwise limit
> > +	 * CPUs to 1, even if multiple vCPUs were configured.
> 
> This is the wrong place as it invalidates the effect of 'nolapic' on the
> kernel command line for XEN PV.
> 
> You actually explain in the comment that XEN disables ACPI, so why are
> you slapping this xen check into this code instead of doing the obvious
> and prevent acpi_mps_check() to cause havoc?

Thank you for your explanation and suggestion. I will correct acpi_mps_check()
in following patch.

Thanks,
Petr