linux-kernel - Re: [PATCH 04/12] swiotlb-xen: ensure to issue well-formed XENMEM

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Date:   Fri, 10 Sep 2021 16:14:59 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Jan Beulich <jbeulich@...e.com>
cc:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH 04/12] swiotlb-xen: ensure to issue well-formed
 XENMEM_exchange requests

On Tue, 7 Sep 2021, Jan Beulich wrote:
> While the hypervisor hasn't been enforcing this, we would still better
> avoid issuing requests with GFNs not aligned to the requested order.
> 
> Signed-off-by: Jan Beulich <jbeulich@...e.com>
> ---
> I wonder how useful it is to include the alignment in the panic()
> message.

Not very useful given that it is static. I don't mind either way but you
can go ahead and remove it if you prefer (and it would make the line
shorter.)


> I further wonder how useful it is to wrap "bytes" in
> PAGE_ALIGN(), when it is a multiple of a segment's size anyway (or at
> least was supposed to be, prior to "swiotlb-xen: maintain slab count
> properly").

This one I would keep, to make sure to print out the same amount passed
to memblock_alloc.


> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -231,10 +231,10 @@ retry:
>  	/*
>  	 * Get IO TLB memory from any location.
>  	 */
> -	start = memblock_alloc(PAGE_ALIGN(bytes), PAGE_SIZE);
> +	start = memblock_alloc(PAGE_ALIGN(bytes), IO_TLB_SEGSIZE << IO_TLB_SHIFT);
>  	if (!start)
> -		panic("%s: Failed to allocate %lu bytes align=0x%lx\n",
> -		      __func__, PAGE_ALIGN(bytes), PAGE_SIZE);
> +		panic("%s: Failed to allocate %lu bytes align=%#x\n",
> +		      __func__, PAGE_ALIGN(bytes), IO_TLB_SEGSIZE << IO_TLB_SHIFT);
>  
>  	/*
>  	 * And replace that memory with pages under 4GB.
>