Re: [PATCH] swiotlb: Allow swiotlb to live at pre-defined address

[Dave Young]

On 03/26/20 at 05:29pm, Alexander Graf wrote:
> The swiotlb is a very convenient fallback mechanism for bounce buffering of
> DMAable data. It is usually used for the compatibility case where devices
> can only DMA to a "low region".
> 
> However, in some scenarios this "low region" may be bound even more
> heavily. For example, there are embedded system where only an SRAM region
> is shared between device and CPU. There are also heterogeneous computing
> scenarios where only a subset of RAM is cache coherent between the
> components of the system. There are partitioning hypervisors, where
> a "control VM" that implements device emulation has limited view into a
> partition's memory for DMA capabilities due to safety concerns.
> 
> This patch adds a command line driven mechanism to move all DMA memory into
> a predefined shared memory region which may or may not be part of the
> physical address layout of the Operating System.
> 
> Ideally, the typical path to set this configuration would be through Device
> Tree or ACPI, but neither of the two mechanisms is standardized yet. Also,
> in the x86 MicroVM use case, we have neither ACPI nor Device Tree, but
> instead configure the system purely through kernel command line options.
> 
> I'm sure other people will find the functionality useful going forward
> though and extend it to be triggered by DT/ACPI in the future.

Hmm, we have a use case for kdump, this maybe useful.  For example
swiotlb is enabled by default if AMD SME/SEV is active, and in kdump
kernel we have to increase the crashkernel reserved size for the extra
swiotlb requirement.  I wonder if we can just reuse the old kernel's
swiotlb region and pass the addr to kdump kernel.

> 
> Signed-off-by: Alexander Graf <graf@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  3 +-
>  Documentation/x86/x86_64/boot-options.rst       |  4 ++-
>  kernel/dma/swiotlb.c                            | 46 +++++++++++++++++++++++--
>  3 files changed, 49 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c07815d230bc..d085d55c3cbe 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4785,11 +4785,12 @@
>  			it if 0 is given (See Documentation/admin-guide/cgroup-v1/memory.rst)
>  
>  	swiotlb=	[ARM,IA-64,PPC,MIPS,X86]
> -			Format: { <int> | force | noforce }
> +			Format: { <int> | force | noforce | addr=<phys addr> }
>  			<int> -- Number of I/O TLB slabs
>  			force -- force using of bounce buffers even if they
>  			         wouldn't be automatically used by the kernel
>  			noforce -- Never use bounce buffers (for debugging)
> +			addr=<phys addr> -- Try to allocate SWIOTLB at defined address
>  
>  	switches=	[HW,M68k]
>  
> diff --git a/Documentation/x86/x86_64/boot-options.rst b/Documentation/x86/x86_64/boot-options.rst
> index 2b98efb5ba7f..ca46c57b68c9 100644
> --- a/Documentation/x86/x86_64/boot-options.rst
> +++ b/Documentation/x86/x86_64/boot-options.rst
> @@ -297,11 +297,13 @@ iommu options only relevant to the AMD GART hardware IOMMU:
>  iommu options only relevant to the software bounce buffering (SWIOTLB) IOMMU
>  implementation:
>  
> -    swiotlb=<pages>[,force]
> +    swiotlb=<pages>[,force][,addr=<phys addr>]
>        <pages>
>          Prereserve that many 128K pages for the software IO bounce buffering.
>        force
>          Force all IO through the software TLB.
> +      addr=<phys addr>
> +        Try to allocate SWIOTLB at defined address
>  
>  Settings for the IBM Calgary hardware IOMMU currently found in IBM
>  pSeries and xSeries machines
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index c19379fabd20..83da0caa2f93 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -46,6 +46,7 @@
>  #include <linux/init.h>
>  #include <linux/memblock.h>
>  #include <linux/iommu-helper.h>
> +#include <linux/io.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/swiotlb.h>
> @@ -102,6 +103,12 @@ unsigned int max_segment;
>  #define INVALID_PHYS_ADDR (~(phys_addr_t)0)
>  static phys_addr_t *io_tlb_orig_addr;
>  
> +/*
> + * The TLB phys addr may be defined on the command line. Store it here if it is.
> + */
> +static phys_addr_t io_tlb_addr = INVALID_PHYS_ADDR;
> +
> +
>  /*
>   * Protect the above data structures in the map and unmap calls
>   */
> @@ -119,11 +126,23 @@ setup_io_tlb_npages(char *str)
>  	}
>  	if (*str == ',')
>  		++str;
> -	if (!strcmp(str, "force")) {
> +	if (!strncmp(str, "force", 5)) {
>  		swiotlb_force = SWIOTLB_FORCE;
> -	} else if (!strcmp(str, "noforce")) {
> +		str += 5;
> +	} else if (!strncmp(str, "noforce", 7)) {
>  		swiotlb_force = SWIOTLB_NO_FORCE;
>  		io_tlb_nslabs = 1;
> +		str += 7;
> +	}
> +
> +	if (*str == ',')
> +		++str;
> +	if (!strncmp(str, "addr=", 5)) {
> +		char *addrstr = str + 5;
> +
> +		io_tlb_addr = kstrtoul(addrstr, 0, &str);
> +		if (addrstr == str)
> +			io_tlb_addr = INVALID_PHYS_ADDR;
>  	}
>  
>  	return 0;
> @@ -239,6 +258,25 @@ int __init swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose)
>  	return 0;
>  }
>  
> +static int __init swiotlb_init_io(int verbose, unsigned long bytes)
> +{
> +	unsigned __iomem char *vstart;
> +
> +	if (io_tlb_addr == INVALID_PHYS_ADDR)
> +		return -EINVAL;
> +
> +	vstart = memremap(io_tlb_addr, bytes, MEMREMAP_WB);
> +	if (!vstart)
> +		return -EINVAL;
> +
> +	if (swiotlb_init_with_tbl(vstart, io_tlb_nslabs, verbose)) {
> +		memunmap(vstart);
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
>  /*
>   * Statically reserve bounce buffer space and initialize bounce buffer data
>   * structures for the software IO TLB used to implement the DMA API.
> @@ -257,6 +295,10 @@ swiotlb_init(int verbose)
>  
>  	bytes = io_tlb_nslabs << IO_TLB_SHIFT;
>  
> +	/* Map IO TLB from device memory */
> +	if (!swiotlb_init_io(verbose, bytes))
> +		return;
> +
>  	/* Get IO TLB memory from the low pages */
>  	vstart = memblock_alloc_low(PAGE_ALIGN(bytes), PAGE_SIZE);
>  	if (vstart && !swiotlb_init_with_tbl(vstart, io_tlb_nslabs, verbose))
> -- 
> 2.16.4
> 
> 
> 
> 
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> 

Thanks
Dave