linux-kernel - Re: [PATCH 6/6 V2] Add age out of guest paused flag

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Message-ID: <4EB04F5F.4010709@us.ibm.com>
Date:	Tue, 01 Nov 2011 14:58:23 -0500
From:	Anthony Liguori <aliguori@...ibm.com>
To:	Eric B Munson <emunson@...bm.net>
CC:	avi@...hat.com, mingo@...hat.com, x86@...nel.org, hpa@...or.com,
	arnd@...db.de, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-arch@...r.kernel.org, ryanh@...ux.vnet.ibm.com
Subject: Re: [PATCH 6/6 V2] Add age out of guest paused flag

On 11/01/2011 02:51 PM, Eric B Munson wrote:
> On Tue, 01 Nov 2011, Anthony Liguori wrote:
>
>> On 10/31/2011 03:07 PM, Eric B Munson wrote:
>>> The KVM_GUEST_PAUSED flag will prevent a guest from compaining about a soft
>>> lockup but it can mask real soft lockups if the flag isn't cleared when it is
>>> no longer relevant.  This patch adds a kvm ioctl that the hypervisor will use
>>> when it resumes a guest to start a timer for aging out the flag.  The time out
>>> will be specified by the hypervisor in the ioctl call.
>>>
>>> Signed-off-by: Eric B Munson<emunson@...bm.net>
>>
>> Why not have the guest clear the flag when it acknowledges it?
>>
>> The hypervisor would unconditionally set the bit, and the guest
>> would do a testandclear to check if the bit is set.  I think that
>> avoids the whole aging business.
>>
>> Regards,
>>
>> Anthony Liguori
>
> If you have a look at patch 5 of this series, the flag is cleared when the
> guest checks the validity of a soft lockup.  However, the aging is meant to
> cover the case where the guest never sees a soft lockup.  We don't want this
> flag to be stored for ever and end up delaying real soft lockup messages.  With
> that case in mind, I thought this was a good/simple compramise.

If the guest clears the flag, then I don't think you have to worry about this.

Regards,

Anthony Liguori

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