RE: [PATCH 10/12] swiotlb: add a SWIOTLB_ANY flag to lift the low memory restriction

["Michael Kelley (LINUX)" <mikelley@xxxxxxxxxxxxx>]

From: Christoph Hellwig <hch@xxxxxx> Sent: Tuesday, March 1, 2022 2:53 AM
> 
> Power SVM wants to allocate a swiotlb buffer that is not restricted to low 
> memory for
> the trusted hypervisor scheme.  Consolidate the support for this into the 
> swiotlb_init
> interface by adding a new flag.

Hyper-V Isolated VMs want to do the same thing of not restricting the swiotlb
buffer to low memory.  That's what Tianyu Lan's patch set[1] is proposing.
Hyper-V synthetic devices have no DMA addressing limitations, and the
likelihood of using a PCI pass-thru device with addressing limitations in an
Isolated VM seems vanishingly small.

So could use of the SWIOTLB_ANY flag be generalized?  Let Hyper-V init
code set the flag before swiotlb_init() is called.  Or provide a CONFIG
variable that Hyper-V Isolated VMs could set.

Michael

[1] https://lore.kernel.org/lkml/20220209122302.213882-1-ltykernel@xxxxxxxxx/

> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  arch/powerpc/include/asm/svm.h       |  4 ----
>  arch/powerpc/include/asm/swiotlb.h   |  1 +
>  arch/powerpc/kernel/dma-swiotlb.c    |  1 +
>  arch/powerpc/mm/mem.c                |  5 +----
>  arch/powerpc/platforms/pseries/svm.c | 26 +-------------------------
>  include/linux/swiotlb.h              |  1 +
>  kernel/dma/swiotlb.c                 |  9 +++++++--
>  7 files changed, 12 insertions(+), 35 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/svm.h b/arch/powerpc/include/asm/svm.h
> index 7546402d796af..85580b30aba48 100644
> --- a/arch/powerpc/include/asm/svm.h
> +++ b/arch/powerpc/include/asm/svm.h
> @@ -15,8 +15,6 @@ static inline bool is_secure_guest(void)
>       return mfmsr() & MSR_S;
>  }
> 
> -void __init svm_swiotlb_init(void);
> -
>  void dtl_cache_ctor(void *addr);
>  #define get_dtl_cache_ctor() (is_secure_guest() ? dtl_cache_ctor : NULL)
> 
> @@ -27,8 +25,6 @@ static inline bool is_secure_guest(void)
>       return false;
>  }
> 
> -static inline void svm_swiotlb_init(void) {}
> -
>  #define get_dtl_cache_ctor() NULL
> 
>  #endif /* CONFIG_PPC_SVM */
> diff --git a/arch/powerpc/include/asm/swiotlb.h
> b/arch/powerpc/include/asm/swiotlb.h
> index 3c1a1cd161286..4203b5e0a88ed 100644
> --- a/arch/powerpc/include/asm/swiotlb.h
> +++ b/arch/powerpc/include/asm/swiotlb.h
> @@ -9,6 +9,7 @@
>  #include <linux/swiotlb.h>
> 
>  extern unsigned int ppc_swiotlb_enable;
> +extern unsigned int ppc_swiotlb_flags;
> 
>  #ifdef CONFIG_SWIOTLB
>  void swiotlb_detect_4g(void);
> diff --git a/arch/powerpc/kernel/dma-swiotlb.c b/arch/powerpc/kernel/dma-
> swiotlb.c
> index fc7816126a401..ba256c37bcc0f 100644
> --- a/arch/powerpc/kernel/dma-swiotlb.c
> +++ b/arch/powerpc/kernel/dma-swiotlb.c
> @@ -10,6 +10,7 @@
>  #include <asm/swiotlb.h>
> 
>  unsigned int ppc_swiotlb_enable;
> +unsigned int ppc_swiotlb_flags;
> 
>  void __init swiotlb_detect_4g(void)
>  {
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c index
> e1519e2edc656..a4d65418c30a9 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -249,10 +249,7 @@ void __init mem_init(void)
>        * back to to-down.
>        */
>       memblock_set_bottom_up(true);
> -     if (is_secure_guest())
> -             svm_swiotlb_init();
> -     else
> -             swiotlb_init(ppc_swiotlb_enable, 0);
> +     swiotlb_init(ppc_swiotlb_enable, ppc_swiotlb_flags);
>  #endif
> 
>       high_memory = (void *) __va(max_low_pfn * PAGE_SIZE); diff --git
> a/arch/powerpc/platforms/pseries/svm.c b/arch/powerpc/platforms/pseries/svm.c
> index c5228f4969eb2..3b4045d508ec8 100644
> --- a/arch/powerpc/platforms/pseries/svm.c
> +++ b/arch/powerpc/platforms/pseries/svm.c
> @@ -28,7 +28,7 @@ static int __init init_svm(void)
>        * need to use the SWIOTLB buffer for DMA even if dma_capable() says
>        * otherwise.
>        */
> -     swiotlb_force = SWIOTLB_FORCE;
> +     ppc_swiotlb_flags |= SWIOTLB_ANY | SWIOTLB_FORCE;
> 
>       /* Share the SWIOTLB buffer with the host. */
>       swiotlb_update_mem_attributes();
> @@ -37,30 +37,6 @@ static int __init init_svm(void)  }  
> machine_early_initcall(pseries,
> init_svm);
> 
> -/*
> - * Initialize SWIOTLB. Essentially the same as swiotlb_init(), except that it
> - * can allocate the buffer anywhere in memory. Since the hypervisor doesn't 
> have
> - * any addressing limitation, we don't need to allocate it in low addresses.
> - */
> -void __init svm_swiotlb_init(void)
> -{
> -     unsigned char *vstart;
> -     unsigned long bytes, io_tlb_nslabs;
> -
> -     io_tlb_nslabs = (swiotlb_size_or_default() >> IO_TLB_SHIFT);
> -     io_tlb_nslabs = ALIGN(io_tlb_nslabs, IO_TLB_SEGSIZE);
> -
> -     bytes = io_tlb_nslabs << IO_TLB_SHIFT;
> -
> -     vstart = memblock_alloc(PAGE_ALIGN(bytes), PAGE_SIZE);
> -     if (vstart && !swiotlb_init_with_tbl(vstart, io_tlb_nslabs, false))
> -             return;
> -
> -
> -     memblock_free(vstart, PAGE_ALIGN(io_tlb_nslabs << IO_TLB_SHIFT));
> -     panic("SVM: Cannot allocate SWIOTLB buffer");
> -}
> -
>  int set_memory_encrypted(unsigned long addr, int numpages)  {
>       if (!cc_platform_has(CC_ATTR_MEM_ENCRYPT))
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h index
> dcecf953f7997..ee655f2e4d28b 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -15,6 +15,7 @@ struct scatterlist;
> 
>  #define SWIOTLB_VERBOSE      (1 << 0) /* verbose initialization */
>  #define SWIOTLB_FORCE        (1 << 1) /* force bounce buffering */
> +#define SWIOTLB_ANY  (1 << 2) /* allow any memory for the buffer */
> 
>  /*
>   * Maximum allowable number of contiguous slabs to map, diff --git
> a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c index 
> 1a40c71c4d51a..77cf73dc20a78
> 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -275,8 +275,13 @@ void __init swiotlb_init(bool addressing_limit, unsigned 
> int
> flags)
>       if (swiotlb_force_disable)
>               return;
> 
> -     /* Get IO TLB memory from the low pages */
> -     tlb = memblock_alloc_low(bytes, PAGE_SIZE);
> +     /*
> +      * By default allocate the bonuce buffer memory from low memory.
> +      */
> +     if (flags & SWIOTLB_ANY)
> +             tlb = memblock_alloc(bytes, PAGE_SIZE);
> +     else
> +             tlb = memblock_alloc_low(bytes, PAGE_SIZE);
>       if (!tlb)
>               goto fail;
>       if (swiotlb_init_with_tbl(tlb, default_nslabs, flags))
> --
> 2.30.2