linux-kernel - Re: [PATCH v3] iscsi_ibft: Fix finding the iBFT under Xen Dom 0

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Date:   Fri, 9 Jun 2023 17:08:32 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Ross Lagerwall <ross.lagerwall@...rix.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Cc:     Jan Beulich <jbeulich@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Peter Jones <pjones@...hat.com>,
        Konrad Rzeszutek Wilk <konrad@...nel.org>
Subject: Re: [PATCH v3] iscsi_ibft: Fix finding the iBFT under Xen Dom 0

On 05.06.23 12:28, Ross Lagerwall wrote:
> To facilitate diskless iSCSI boot, the firmware can place a table of
> configuration details in memory called the iBFT. The presence of this
> table is not specified, nor is the precise location (and it's not in the
> E820) so the kernel has to search for a magic marker to find it.
> 
> When running under Xen, Dom 0 does not have access to the entire host's
> memory, only certain regions which are identity-mapped which means that
> the pseudo-physical address in Dom0 == real host physical address.
> Add the iBFT search bounds as a reserved region which causes it to be
> identity-mapped in xen_set_identity_and_remap_chunk() which allows Dom0
> access to the specific physical memory to correctly search for the iBFT
> magic marker (and later access the full table).
> 
> This necessitates moving the call to reserve_ibft_region() somewhat
> later so that it is called after e820__memory_setup() which is when the
> Xen identity mapping adjustments are applied. The precise location of
> the call is not too important so I've put it alongside dmi_setup() which
> does similar scanning of memory for configuration tables.
> 
> Finally in the iBFT find code, instead of using isa_bus_to_virt() which
> doesn't do the right thing under Xen, use early_memremap() like the
> dmi_setup() code does.
> 
> The result of these changes is that it is possible to boot a diskless
> Xen + Dom0 running off an iSCSI disk whereas previously it would fail to
> find the iBFT and consequently, the iSCSI root disk.
> 
> Signed-off-by: Ross Lagerwall <ross.lagerwall@...rix.com>

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen


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