linux-kernel - Re: [PATCH v13 01/12] swiotlb: Refactor swiotlb init functions

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Date:   Mon, 21 Jun 2021 10:59:20 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Christoph Hellwig <hch@....de>
cc:     Tom Lendacky <thomas.lendacky@....com>,
        Claire Chang <tientzu@...omium.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Rob Herring <robh+dt@...nel.org>, mpe@...erman.id.au,
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Subject: Re: [PATCH v13 01/12] swiotlb: Refactor swiotlb init functions

On Fri, 18 Jun 2021, Christoph Hellwig wrote:
> On Fri, Jun 18, 2021 at 09:09:17AM -0500, Tom Lendacky wrote:
> > > swiotlb_init_with_tbl uses memblock_alloc to allocate the io_tlb_mem
> > > and memblock_alloc[1] will do memset in memblock_alloc_try_nid[2], so
> > > swiotlb_init_with_tbl is also good.
> > > I'm happy to add the memset in swiotlb_init_io_tlb_mem if you think
> > > it's clearer and safer.
> > 
> > On x86, if the memset is done before set_memory_decrypted() and memory
> > encryption is active, then the memory will look like ciphertext afterwards
> > and not be zeroes. If zeroed memory is required, then a memset must be
> > done after the set_memory_decrypted() calls.
> 
> Which should be fine - we don't care that the memory is cleared to 0,
> just that it doesn't leak other data.  Maybe a comment would be useful,
> though,

Just as a clarification: I was referring to the zeroing of "mem" in
swiotlb_late_init_with_tbl and swiotlb_init_with_tbl. While it looks
like Tom and Christoph are talking about the zeroing of "tlb".

The zeroing of "mem" is required as some fields of struct io_tlb_mem
need to be initialized to zero. While the zeroing of "tlb" is not
required except from the point of view of not leaking sensitive data as
Christoph mentioned.

Either way, Claire's v14 patch 01/12 looks fine to me.