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Subject: Re: [PATCH v2] powerpc/32s: Fix DSI and ISI exceptions for
 CONFIG_VMAP_STACK
From: Christophe Leroy <christophe.leroy@c-s.fr>
To: linuxppc-dev@lists.ozlabs.org
References: <6981d1be4504fc25b893f7127611e74c224887eb.1581588239.git.christophe.leroy@c-s.fr>
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Cc: "Erhard F." <erhard_f@mailbox.org>, linux-kernel@vger.kernel.org
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Le 13/02/2020 à 11:04, Christophe Leroy a écrit :
> hash_page() needs to read page tables from kernel memory. When entire
> kernel memory is mapped by BATs, which is normally the case when
> CONFIG_STRICT_KERNEL_RWX is not set, it works even if the page hosting
> the page table is not referenced in the MMU hash table.
> 
> However, if the page where the page table resides is not covered by
> a BAT, a DSI fault can be encountered from hash_page(), and it loops
> forever. This can happen when CONFIG_STRICT_KERNEL_RWX is selected
> and the alignment of the different regions is too small to allow
> covering the entire memory with BATs. This also happens when
> CONFIG_DEBUG_PAGEALLOC is selected or when booting with 'nobats'
> flag.
> 
> Also, if the page containing the kernel stack is not present in the
> MMU hash table, registers cannot be saved and a recursive DSI fault
> is encountered.
> 
> To allow hash_page() to properly do its job at all time and load the
> MMU hash table whenever needed, it must run with data MMU disabled.
> This means it must be called before re-enabling data MMU. To allow
> this, registers clobbered by hash_page() and create_hpte() have to
> be saved in the thread struct together with SRR0, SSR1, DAR and DSISR.
> It is also necessary to ensure that DSI prolog doesn't overwrite
> regs saved by prolog of the current running exception. That means:
> - DSI can only use SPRN_SPRG_SCRATCH0
> - Exceptions must free SPRN_SPRG_SCRATCH0 before writing to the stack.
> 
> This also fixes the Oops reported by Erhard when create_hpte() is
> called by add_hash_page().
> 
> Fixes: cd08f109e262 ("powerpc/32s: Enable CONFIG_VMAP_STACK")
> Reported-by: Erhard F. <erhard_f@mailbox.org>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206501
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>

Tested-by: Erhard F. <erhard_f@mailbox.org>