linux-kernel - RE: [PATCH v2] hv: retry infinitely on hypercall transient failures

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Date:   Sat, 7 Jan 2017 07:23:14 +0000
From:   Long Li <longli@...rosoft.com>
To:     Greg KH <greg@...ah.com>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] hv: retry infinitely on hypercall transient failures

> -----Original Message-----
> From: Greg KH [mailto:greg@...ah.com]
> Sent: Wednesday, January 04, 2017 11:48 PM
> To: Long Li <longli@...rosoft.com>
> Cc: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> <haiyangz@...rosoft.com>; devel@...uxdriverproject.org; linux-
> kernel@...r.kernel.org
> Subject: Re: [PATCH v2] hv: retry infinitely on hypercall transient failures
> 
> On Wed, Jan 04, 2017 at 06:12:20PM -0800, Long Li wrote:
> > From: Long Li <longli@...rosoft.com>
> >
> > Hyper-v host guarantees that a hypercall will finish in reasonable time.
> > Retry infinitely on transient failures to avoid returning error to upper layer.
> 
> Again, never retry "forever", always have a way out, otherwise you will crash.
> 
> And again, why are you making this change?  What problem does it solve?

The problem it tries to solve is that in this code we are returning error prematurely on transient failures. The hypercall is used mostly in channel establishment. If we return a transient failure, the VM may not boot or not useful after boot due to some devices missing.

Another approach is to increase the number of retries. But we don't know how many retries is safe, and Windows host side expects the guest retry infinitely and not return error on transient failures.

> 
> greg k-h