linux-kernel - Re: [RFC PATCH v3 1/6] swiotlb: Add io_tlb

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Date:   Wed, 13 Jan 2021 12:50:31 +0100
From:   Christoph Hellwig <hch@....de>
To:     Claire Chang <tientzu@...omium.org>
Cc:     robh+dt@...nel.org, mpe@...erman.id.au, benh@...nel.crashing.org,
        paulus@...ba.org, joro@...tes.org, will@...nel.org,
        frowand.list@...il.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        sstabellini@...nel.org, hch@....de, m.szyprowski@...sung.com,
        robin.murphy@....com, grant.likely@....com, xypron.glpk@....de,
        treding@...dia.com, mingo@...nel.org, bauerman@...ux.ibm.com,
        peterz@...radead.org, gregkh@...uxfoundation.org,
        saravanak@...gle.com, rafael.j.wysocki@...el.com,
        heikki.krogerus@...ux.intel.com, andriy.shevchenko@...ux.intel.com,
        rdunlap@...radead.org, dan.j.williams@...el.com,
        bgolaszewski@...libre.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        iommu@...ts.linux-foundation.org, xen-devel@...ts.xenproject.org,
        tfiga@...omium.org, drinkcat@...omium.org
Subject: Re: [RFC PATCH v3 1/6] swiotlb: Add io_tlb_mem struct

On Wed, Jan 06, 2021 at 11:41:19AM +0800, Claire Chang wrote:
> Added a new struct, io_tlb_mem, as the IO TLB memory pool descriptor and
> moved relevant global variables into that struct.
> This will be useful later to allow for restricted DMA pool.

I like where this is going, but a few comments.

Mostly I'd love to be able to entirely hide io_tlb_default_mem
and struct io_tlb_mem inside of swiotlb.c.

> --- a/arch/powerpc/platforms/pseries/svm.c
> +++ b/arch/powerpc/platforms/pseries/svm.c
> @@ -55,8 +55,8 @@ void __init svm_swiotlb_init(void)
>  	if (vstart && !swiotlb_init_with_tbl(vstart, io_tlb_nslabs, false))
>  		return;
>  
> -	if (io_tlb_start)
> -		memblock_free_early(io_tlb_start,
> +	if (io_tlb_default_mem.start)
> +		memblock_free_early(io_tlb_default_mem.start,
>  				    PAGE_ALIGN(io_tlb_nslabs << IO_TLB_SHIFT));

I think this should switch to use the local vstart variable in
prep patch.

>  	panic("SVM: Cannot allocate SWIOTLB buffer");
>  }
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index 2b385c1b4a99..4d17dff7ffd2 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -192,8 +192,8 @@ int __ref xen_swiotlb_init(int verbose, bool early)
>  	/*
>  	 * IO TLB memory already allocated. Just use it.
>  	 */
> -	if (io_tlb_start != 0) {
> -		xen_io_tlb_start = phys_to_virt(io_tlb_start);
> +	if (io_tlb_default_mem.start != 0) {
> +		xen_io_tlb_start = phys_to_virt(io_tlb_default_mem.start);
>  		goto end;

xen_io_tlb_start is interesting. It is used only in two functions:

 1) is_xen_swiotlb_buffer, where I think we should be able to just use
    is_swiotlb_buffer instead of open coding it with the extra
    phys_to_virt/virt_to_phys cycle.
 2) xen_swiotlb_init, where except for the assignment it only is used
    locally for the case not touched above and could this be replaced
    with a local variable.

Konrad, does this make sense to you?

>  static inline bool is_swiotlb_buffer(phys_addr_t paddr)
>  {
> -	return paddr >= io_tlb_start && paddr < io_tlb_end;
> +	struct io_tlb_mem *mem = &io_tlb_default_mem;
> +
> +	return paddr >= mem->start && paddr < mem->end;

We'd then have to move this out of line as well.