linux-kernel - Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped by host

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: Windows password security audit tool. GUI, reports in PDF.

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

Date:	Wed, 14 Dec 2011 10:16:22 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Eric B Munson <emunson@...bm.net>, mingo@...hat.com, hpa@...or.com,
	arnd@...db.de, ryanh@...ux.vnet.ibm.com, aliguori@...ibm.com,
	jeremy.fitzhardinge@...rix.com, levinsasha928@...il.com,
	Jan Kiszka <jan.kiszka@...mens.com>, kvm@...r.kernel.org,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped by
 host

On Wed, Dec 07, 2011 at 04:41:17PM +0200, Avi Kivity wrote:
> On 12/05/2011 10:18 PM, Eric B Munson wrote:
> > Changes from V4:
> > Rename KVM_GUEST_PAUSED to KVMCLOCK_GUEST_PAUSED
> > Add description of KVMCLOCK_GUEST_PAUSED ioctl to api.txt
> >
> > Changes from V3:
> > Include CC's on patch 3
> > Drop clear flag ioctl and have the watchdog clear the flag when it is reset
> >
> > Changes from V2:
> > A new kvm functions defined in kvm_para.h, the only change to pvclock is the
> > initial flag definition
> >
> > Changes from V1:
> > (Thanks Marcelo)
> > Host code has all been moved to arch/x86/kvm/x86.c
> > KVM_PAUSE_GUEST was renamed to KVM_GUEST_PAUSED
> >
> > When a guest kernel is stopped by the host hypervisor it can look like a soft
> > lockup to the guest kernel.  This false warning can mask later soft lockup
> > warnings which may be real.  This patch series adds a method for a host
> > hypervisor to communicate to a guest kernel that it is being stopped.  The
> > final patch in the series has the watchdog check this flag when it goes to
> > issue a soft lockup warning and skip the warning if the guest knows it was
> > stopped.
> >
> > It was attempted to solve this in Qemu, but the side effects of saving and
> > restoring the clock and tsc for each vcpu put the wall clock of the guest behind
> > by the amount of time of the pause.  This forces a guest to have ntp running
> > in order to keep the wall clock accurate.
> 
> Having this controlled from userspace means it doesn't work for SIGSTOP
> or for long scheduling delays.  What about doing this automatically
> based on preempt notifiers?

Long scheduling delays should be considered hangups from the guest
perspective.

About SIGSTOP, that is a corner case. Unsure if its even properly supported 
by QEMU.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/