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SubjectRe: [RFC v1 26/26] x86/kvm: Use bounce buffers for TD guest
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On 2/5/21 3:38 PM, Kuppuswamy Sathyanarayanan wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>
> TDX doesn't allow to perform DMA access to guest private memory.
> In order for DMA to work properly in TD guest, user SWIOTLB bounce
> buffers.
>
> Move AMD SEV initialization into common code and adopt for TDX.

This would be best if it can draw a parallel between TDX and SEV.

> arch/x86/kernel/pci-swiotlb.c | 2 +-
> arch/x86/kernel/tdx.c | 3 +++
> arch/x86/mm/mem_encrypt.c | 44 -------------------------------
> arch/x86/mm/mem_encrypt_common.c | 45 ++++++++++++++++++++++++++++++++
> 4 files changed, 49 insertions(+), 45 deletions(-)
>
> diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
> index c2cfa5e7c152..020e13749758 100644
> --- a/arch/x86/kernel/pci-swiotlb.c
> +++ b/arch/x86/kernel/pci-swiotlb.c
> @@ -49,7 +49,7 @@ int __init pci_swiotlb_detect_4gb(void)
> * buffers are allocated and used for devices that do not support
> * the addressing range required for the encryption mask.
> */
> - if (sme_active())
> + if (sme_active() || is_tdx_guest())
> swiotlb = 1;
>
> return swiotlb;
> diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
> index f51a19168adc..ccb9401bd706 100644
> --- a/arch/x86/kernel/tdx.c
> +++ b/arch/x86/kernel/tdx.c
> @@ -9,6 +9,7 @@
> #include <asm/vmx.h>
> #include <asm/insn.h>
> #include <linux/sched/signal.h> /* force_sig_fault() */
> +#include <linux/swiotlb.h>
>
> #ifdef CONFIG_KVM_GUEST
> #include "tdx-kvm.c"
> @@ -472,6 +473,8 @@ void __init tdx_early_init(void)
>
> legacy_pic = &null_legacy_pic;
>
> + swiotlb_force = SWIOTLB_FORCE;

Dumb question time. But, what is the difference between

swiotlb = 1;

and

swiotlb_force = SWIOTLB_FORCE;

It would be nice of the patch to enable me to be a lazy reviewer.

> cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "tdx:cpu_hotplug",
> NULL, tdx_cpu_offline_prepare);
>
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index 11a6a7b3af7e..7fbbb2f3d426 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c

Should we be renaming this to amd_mem_encrypt.c or something?

...
> - */
> - if (sev_active())
> - static_branch_enable(&sev_enable_key);
> -
> - print_mem_encrypt_feature_info();
> -}
> -
> diff --git a/arch/x86/mm/mem_encrypt_common.c b/arch/x86/mm/mem_encrypt_common.c
> index b6d93b0c5dcf..6f3d90d4d68e 100644
> --- a/arch/x86/mm/mem_encrypt_common.c
> +++ b/arch/x86/mm/mem_encrypt_common.c
> @@ -10,6 +10,7 @@
> #include <linux/mm.h>
> #include <linux/mem_encrypt.h>
> #include <linux/dma-mapping.h>
> +#include <linux/swiotlb.h>
>
> /* Override for DMA direct allocation check - ARCH_HAS_FORCE_DMA_UNENCRYPTED */
> bool force_dma_unencrypted(struct device *dev)
> @@ -36,3 +37,47 @@ bool force_dma_unencrypted(struct device *dev)
>
> return false;
> }
> +
> +static void print_mem_encrypt_feature_info(void)
> +{

This function is now named wrong IMNHO. If it's about AMD only, it
needs AMD in the name.

> + pr_info("AMD Memory Encryption Features active:");
> +
> + /* Secure Memory Encryption */
> + if (sme_active()) {
> + /*
> + * SME is mutually exclusive with any of the SEV
> + * features below.
> + */
> + pr_cont(" SME\n");
> + return;
> + }
> +
> + /* Secure Encrypted Virtualization */
> + if (sev_active())
> + pr_cont(" SEV");
> +
> + /* Encrypted Register State */
> + if (sev_es_active())
> + pr_cont(" SEV-ES");
> +
> + pr_cont("\n");
> +}

I'm really tempted to say this needs to be off in arch/x86/kernel/cpu/amd.c

> +/* Architecture __weak replacement functions */
> +void __init mem_encrypt_init(void)
> +{
> + if (!sme_me_mask && !is_tdx_guest())
> + return;

The direct check of sme_me_mask looks odd now. What does this *MEAN*?
Are we looking to jump out of here if no memory encryption is enabled?

I'd much rather this look more like:

if (!x86_memory_encryption())
return;

> + /* Call into SWIOTLB to update the SWIOTLB DMA buffers */
> + swiotlb_update_mem_attributes();
> + /*
> + * With SEV, we need to unroll the rep string I/O instructions.
> + */
> + if (sev_active())
> + static_branch_enable(&sev_enable_key);
> +
> + if (!is_tdx_guest())
> + print_mem_encrypt_feature_info();
> +}

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