linux-kernel - Re: [PART1 RFC v3 12/12] svm: Manage vcpu load/unload when enable AVIC

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Date:	Thu, 31 Mar 2016 16:19:08 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Cc:	pbonzini@...hat.com, joro@...tes.org, bp@...en8.de,
	gleb@...nel.org, alex.williamson@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, wei@...hat.com,
	sherry.hurwitz@....com
Subject: Re: [PART1 RFC v3 12/12] svm: Manage vcpu load/unload when enable
 AVIC

2016-03-31 15:52+0700, Suravee Suthikulpanit:
> On 03/19/2016 04:44 AM, Radim Krčmář wrote:
>>2016-03-18 01:09-0500, Suravee Suthikulpanit:
>>>+	} else {
>>>+		new_entry = READ_ONCE(*entry);
>>>+		/**
>>>+		 * This handles the case when vcpu is scheduled out
>>>+		 * and has not yet not called blocking. We save the
>>>+		 * AVIC running flag so that we can restore later.
>>>+		 */
>>
>>is_running must be disabled in between ...blocking and ...unblocking,
>>because we don't want to miss interrupts and block forever.
>>I somehow don't get it from the comment. :)
> 
> Not sure if I understand your concern.  However, the is_running bit
> setting/clearing should be handled in the avic_set_running below. This part
> only handles othe case when the is_running bit still set when calling
> vcpu_put (and later on loading some other vcpus). This way, when we are
> re-loading this vcpu, we can restore the is_running bit accordingly.

I think that the comment is misleading.  The saved is_running flag only
matters after svm_vcpu_blocking, yet the comment says that it handles
the irrelevant case before.

Another minor bug is that was_running isn't initialized to 1, so we need
to halt before is_running gets set.
It would be clearer to toggle a flag in svm_vcpu_(un)blocking and set
is_running = !is_blocking.  Doing so will also be immeasurably faster,
because avic_vcpu_load is called far more than svm_vcpu_(un)blocking.