linux-kernel - RE: [PANIC, hyperv] BUG: unable to handle kernel paging request at ffff880077800004 (hv_ringbuffer

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Message-ID: <EE124450C0AAF944A40DD71E61F878C997A1DB@SINEX14MBXC418.southpacific.corp.microsoft.com>
Date:	Fri, 29 Aug 2014 14:48:25 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	Sitsofe Wheeler <sitsofe@...il.com>,
	Dan Carpenter <dan.carpenter@...cle.com>
CC:	KY Srinivasan <kys@...rosoft.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>
Subject: RE: [PANIC, hyperv] BUG: unable to handle kernel paging request at
 ffff880077800004 (hv_ringbuffer_write)

> -----Original Message-----
> From: Sitsofe Wheeler
> Sent: Friday, August 29, 2014 19:20 PM
> Because this particular return code is overloaded with multiple meanings
> that can't necessarily be deduced from its name perhaps this
> documentation patch would be helpful:
> 
> diff --git a/arch/x86/include/uapi/asm/hyperv.h
> b/arch/x86/include/uapi/asm/hyperv.h
> index 462efe7..04894d5 100644
> --- a/arch/x86/include/uapi/asm/hyperv.h
> +++ b/arch/x86/include/uapi/asm/hyperv.h
> @@ -213,6 +213,12 @@
>  #define HV_STATUS_SUCCESS                      0
>  #define HV_STATUS_INVALID_HYPERCALL_CODE       2
>  #define HV_STATUS_INVALID_HYPERCALL_INPUT      3
> +/*
> + * HV_STATUS_INVALID_ALIGNMENT indicates one of the following:
> + *   The input or output pointer is not aligned to 8 bytes.
> + *   The input or output lists span more than one page.
> + *   The input or output pointer is out of bounds.
> + */
>  #define HV_STATUS_INVALID_ALIGNMENT            4
>  #define HV_STATUS_INSUFFICIENT_BUFFERS         19
> 
> Dexuan: Does this mean that if input/output pointer is well aligned and
> the (payload start) is in the same page as (payload start + payload
> size) all will be well? If so it won't matter what the length of payload
> size is right?
> 
> --
> Sitsofe | http://sucs.org/~sits/

Hi Sitsofe,
I personally think so, but I'm not 100% sure about the exact behavior of
the hypervisor here:
my previous patch tried to only use page-aligned-allocation when
payload_start and payload_end are not on the same page, but I still got
the "invalid alignment" error code occasionally, which makes me a little
upset, honestly speaking. 

Thanks,
-- Dexuan
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