linux-kernel - Re: [PATCH v5 6/7] acpi/ghes: add support for generic error injection via QAPI

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Message-ID: <20240807103436.000013fc@Huawei.com>
Date: Wed, 7 Aug 2024 10:34:36 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
CC: Igor Mammedov <imammedo@...hat.com>, Shiju Jose <shiju.jose@...wei.com>,
	"Michael S. Tsirkin" <mst@...hat.com>, Ani Sinha <anisinha@...hat.com>,
	Dongjiu Geng <gengdongjiu1@...il.com>, <linux-kernel@...r.kernel.org>,
	<qemu-arm@...gnu.org>, <qemu-devel@...gnu.org>
Subject: Re: [PATCH v5 6/7] acpi/ghes: add support for generic error
 injection via QAPI

On Wed, 7 Aug 2024 09:47:50 +0200
Mauro Carvalho Chehab <mchehab+huawei@...nel.org> wrote:

> Em Tue, 6 Aug 2024 16:31:13 +0200
> Igor Mammedov <imammedo@...hat.com> escreveu:
> 
> > PS:
> > looking at the code, ACPI_GHES_MAX_RAW_DATA_LENGTH is 1K
> > and it is the total size of a error block for a error source.
> > 
> > However acpi_hest_ghes.rst (3) says it should be 4K,
> > am I mistaken?  
> 
> Maybe Jonathan knows better, but I guess the 1K was just some
> arbitrary limit to prevent a too big CPER. The 4K limit described
> at acpi_hest_ghes.rst could be just some limit to cope with
> the current bios implementation, but I didn't check myself how
> this is implemented there. 
> 
> I was unable to find any limit at the specs. Yet, if you look at:
> 
> https://uefi.org/specs/UEFI/2.10/Apx_N_Common_Platform_Error_Record.html#arm-processor-error-section

I think both limits are just made up.  You can in theory log huge
error records.  Just not one does.

> 
> The processor Error Information Structure, starting at offset
> 40, can go up to 255*32, meaning an offset of 8200, which is
> bigger than 4K.
> 
> Going further, processor context can have up to 65535 (spec
> actually says 65536, but that sounds a typo, as the size is
> stored on an uint16_t), containing multiple register values
> there (the spec calls its length as "P").
> 
> So, the CPER record could, in theory, have:
> 	8200 + (65535 * P) + sizeof(vendor-specicific-info)
> 
> The CPER length is stored in Section Length record, which is
> uint32_t.
> 
> So, I'd say that the GHES record can theoretically be a lot
> bigger than 4K.	
Agreed - but I don't think we care for testing as long as it's
big enough for plausible records.   Unless you really want
to fuzz the limits?

Jonathan

> 
> Thanks,
> Mauro