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

Michael Kelley (LINUX)
2022-02-28T02:53:39Z

From: Christoph Hellwig <hch@lst.de> 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@lst.de>
> ---
> 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