RE: [PATCH V6 3/5] hyper-v: Enable swiotlb bounce buffer for Isolation VM

Michael Kelley (LINUX)
2021-12-09T20:25:28Z

From: Tianyu Lan <ltykernel@gmail.com> Sent: Monday, December 6, 2021 11:56 PM
>
> hyperv Isolation VM requires bounce buffer support to copy
> data from/to encrypted memory and so enable swiotlb force
> mode to use swiotlb bounce buffer for DMA transaction.
>
> In Isolation VM with AMD SEV, the bounce buffer needs to be
> accessed via extra address space which is above shared_gpa_boundary
> (E.G 39 bit address line) reported by Hyper-V CPUID ISOLATION_CONFIG.
> The access physical address will be original physical address +
> shared_gpa_boundary. The shared_gpa_boundary in the AMD SEV SNP
> spec is called virtual top of memory(vTOM). Memory addresses below
> vTOM are automatically treated as private while memory above
> vTOM is treated as shared.
>
> Swiotlb bounce buffer code calls set_memory_decrypted()
> to mark bounce buffer visible to host and map it in extra
> address space via memremap. Populate the shared_gpa_boundary
> (vTOM) via swiotlb_unencrypted_base variable.
>
> The map function memremap() can't work in the early place
> (e.g ms_hyperv_init_platform()) and so call swiotlb_update_mem_
> attributes() in the hyperv_init().
>
> Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
> ---
> Change since v4:
> * Remove Hyper-V IOMMU IOMMU_INIT_FINISH related functions
> and set SWIOTLB_FORCE and swiotlb_unencrypted_base in the
> ms_hyperv_init_platform(). Call swiotlb_update_mem_attributes()
> in the hyperv_init().
>
> Change since v3:
> * Add comment in pci-swiotlb-xen.c to explain why add
> dependency between hyperv_swiotlb_detect() and pci_
> xen_swiotlb_detect().
> * Return directly when fails to allocate Hyper-V swiotlb
> buffer in the hyperv_iommu_swiotlb_init().
> ---
> arch/x86/hyperv/hv_init.c | 10 ++++++++++
> arch/x86/kernel/cpu/mshyperv.c | 11 ++++++++++-
> include/linux/hyperv.h | 8 ++++++++
> 3 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 24f4a06ac46a..9e18a280f89d 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -28,6 +28,7 @@
> #include <linux/syscore_ops.h>
> #include <clocksource/hyperv_timer.h>
> #include <linux/highmem.h>
> +#include <linux/swiotlb.h>
>
> int hyperv_init_cpuhp;
> u64 hv_current_partition_id = ~0ull;
> @@ -502,6 +503,15 @@ void __init hyperv_init(void)
>
> /* Query the VMs extended capability once, so that it can be cached. */
> hv_query_ext_cap(0);
> +
> + /*
> + * Swiotlb bounce buffer needs to be mapped in extra address
> + * space. Map function doesn't work in the early place and so
> + * call swiotlb_update_mem_attributes() here.
> + */
> + if (hv_is_isolation_supported())
> + swiotlb_update_mem_attributes();
> +
> return;
>
> clean_guest_os_id:
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 4794b716ec79..baf3a0873552 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -18,6 +18,7 @@
> #include <linux/kexec.h>
> #include <linux/i8253.h>
> #include <linux/random.h>
> +#include <linux/swiotlb.h>
> #include <asm/processor.h>
> #include <asm/hypervisor.h>
> #include <asm/hyperv-tlfs.h>
> @@ -319,8 +320,16 @@ static void __init ms_hyperv_init_platform(void)
> pr_info("Hyper-V: Isolation Config: Group A 0x%x, Group B 0x%x\n",
> ms_hyperv.isolation_config_a, ms_hyperv.isolation_config_b);
>
> - if (hv_get_isolation_type() == HV_ISOLATION_TYPE_SNP)
> + if (hv_get_isolation_type() == HV_ISOLATION_TYPE_SNP) {
> static_branch_enable(&isolation_type_snp);
> + swiotlb_unencrypted_base = ms_hyperv.shared_gpa_boundary;
> + }
> +
> + /*
> + * Enable swiotlb force mode in Isolation VM to
> + * use swiotlb bounce buffer for dma transaction.
> + */
> + swiotlb_force = SWIOTLB_FORCE;

I'm good with this approach that directly updates the swiotlb settings here
rather than in IOMMU initialization code. It's a lot more straightforward.

However, there's an issue if building for X86_32 without PAE, in that the
swiotlb module may not be built, resulting in compile and link errors. The
swiotlb.h file needs to be updated to provide a stub function for
swiotlb_update_mem_attributes(). swiotlb_unencrypted_base probably
needs wrapper functions to get/set it, which can be stubs when
CONFIG_SWIOTLB is not set. swiotlb_force is a bit of a mess in that it already
has a stub definition that assumes it will only be read, and not set. A bit of
thinking will be needed to sort that out.

> }
>
> if (hv_max_functions_eax >= HYPERV_CPUID_NESTED_FEATURES) {
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index b823311eac79..1f037e114dc8 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -1726,6 +1726,14 @@ int hyperv_write_cfg_blk(struct pci_dev *dev, void *buf, unsigned int len,
> int hyperv_reg_block_invalidate(struct pci_dev *dev, void *context,
> void (*block_invalidate)(void *context,
> u64 block_mask));
> +#if IS_ENABLED(CONFIG_HYPERV)
> +int __init hyperv_swiotlb_detect(void);
> +#else
> +static inline int __init hyperv_swiotlb_detect(void)
> +{
> + return 0;
> +}
> +#endif

I don't think hyperv_swiotlb_detect() is used any longer, so this change
should be dropped.

>
> struct hyperv_pci_block_ops {
> int (*read_block)(struct pci_dev *dev, void *buf, unsigned int buf_len,
> --
> 2.25.1