RE: [PATCH 08/11] swiotlb: make the swiotlb_init interface more useful

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

From: Christoph Hellwig <hch@xxxxxx> Sent: Sunday, February 27, 2022 6:31 AM
> 
> Pass a bool to pass if swiotlb needs to be enabled based on the
> addressing needs and replace the verbose argument with a set of
> flags, including one to force enable bounce buffering.
> 
> Note that this patch removes the possibility to force xen-swiotlb
> use using swiotlb=force on the command line on x86 (arm and arm64
> never supported that), but this interface will be restored shortly.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  arch/arm/mm/init.c                     |  6 +----
>  arch/arm64/mm/init.c                   |  6 +----
>  arch/ia64/mm/init.c                    |  4 +--
>  arch/mips/cavium-octeon/dma-octeon.c   |  2 +-
>  arch/mips/loongson64/dma.c             |  2 +-
>  arch/mips/sibyte/common/dma.c          |  2 +-
>  arch/powerpc/include/asm/swiotlb.h     |  1 +
>  arch/powerpc/mm/mem.c                  |  3 ++-
>  arch/powerpc/platforms/pseries/setup.c |  3 ---
>  arch/riscv/mm/init.c                   |  8 +-----
>  arch/s390/mm/init.c                    |  3 +--
>  arch/x86/kernel/cpu/mshyperv.c         |  8 ------
>  arch/x86/kernel/pci-dma.c              | 15 ++++++-----
>  arch/x86/mm/mem_encrypt_amd.c          |  3 ---
>  drivers/xen/swiotlb-xen.c              |  4 +--
>  include/linux/swiotlb.h                | 15 ++++++-----
>  include/trace/events/swiotlb.h         | 29 ++++++++-------------
>  kernel/dma/swiotlb.c                   | 35 ++++++++++++++------------
>  18 files changed, 56 insertions(+), 93 deletions(-)

[snip]

> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 5a99f993e6392..568274917f1cd 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -336,14 +336,6 @@ static void __init ms_hyperv_init_platform(void)
>                       swiotlb_unencrypted_base = 
> ms_hyperv.shared_gpa_boundary;
>  #endif
>               }
> -
> -#ifdef CONFIG_SWIOTLB
> -             /*
> -              * Enable swiotlb force mode in Isolation VM to
> -              * use swiotlb bounce buffer for dma transaction.
> -              */
> -             swiotlb_force = SWIOTLB_FORCE;
> -#endif

With this code removed, it's not clear to me what forces the use of the
swiotlb in a Hyper-V isolated VM.  The code in pci_swiotlb_detect_4g() doesn't
catch this case because cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT)
returns "false" in a Hyper-V guest.  In the Hyper-V guest, it's only
cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) that returns "true".  I'm
looking more closely at the meaning of the CC_ATTR_* values, and it may
be that Hyper-V should also return "true" for CC_ATTR_MEM_ENCRYPT,
but I don't think CC_ATTR_HOST_MEM_ENCRYPT should return "true".

Michael