Re: [PATCH v8 4/5] x86/mm: Add noalias variants of set_memory_*crypted() functions

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Mon, Jul 18, 2022 at 07:22:52AM -0700, Sathyanarayanan Kuppuswamy wrote:
> Hi Dave,
> 
> On 7/5/22 8:29 AM, Kirill A. Shutemov wrote:
> >>> I still don't like the idea of using the DMA API itself.  But, maybe we
> >>> need some common infrastructure that the DMA API and this code use which
> >>> says, "get me some pages that I can safely make shared".
> >> Right.  For instance any KVM PV feature would require shared memory, and DMA API
> >> normally doesn't fit (taking 'struct kvm_steal_time' as example).
> >>
> >> Maybe we can reserve a CMA for this purpose.
> > CMA for couple low traffic users sounds like an overkill. It will create
> > an separate pool just for them.
> > 
> > I think the best way is to add an dummy device and couple of helpers
> > around DMA API that would allow to tap into swiotlb.
> > 
> > Maybe hide it inside CC infrastructure. Like cc_decrypted_alloc() and
> > cc_decrypted_free().
> 
> I also think creating a generic device in the CC layer, and using it to allocate
> memory via DMA APIs is a better approach for this issue. Following is the sample
> implementation to give you an idea on how it looks. Please let me know
> your comments.
> 
> cc_shmem_alloc/free() APIs can be used by CC users to allocate and
> free shared memory.

We usually use 'decrypted' term in kernel. cc_decrypted_alloc()/_free().

> Other vendors can define their own shared memory allocation and free
> logic via struct shmem_priv alloc/free/init hooks.
> 
> --- a/arch/x86/coco/Makefile
> +++ b/arch/x86/coco/Makefile
> @@ -3,6 +3,6 @@ CFLAGS_REMOVE_core.o    = -pg
>  KASAN_SANITIZE_core.o  := n
>  CFLAGS_core.o          += -fno-stack-protector
>  
> -obj-y += core.o
> +obj-y += core.o shmem.o

Rename shmem.o -> mem.o ?

> diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
> index 49b44f881484..62fe68d1f60a 100644
> --- a/arch/x86/coco/core.c
> +++ b/arch/x86/coco/core.c
> @@ -13,7 +13,7 @@
>  #include <asm/coco.h>
>  #include <asm/processor.h>
>  
> -static enum cc_vendor vendor __ro_after_init;
> +enum cc_vendor vendor __ro_after_init;
>  static u64 cc_mask __ro_after_init;
>  
>  static bool intel_cc_platform_has(enum cc_attr attr)
> @@ -23,6 +23,7 @@ static bool intel_cc_platform_has(enum cc_attr attr)
>         case CC_ATTR_HOTPLUG_DISABLED:
>         case CC_ATTR_GUEST_MEM_ENCRYPT:
>         case CC_ATTR_MEM_ENCRYPT:
> +       case CC_ATTR_SHMEM:
>                 return true;
>         default:
>                 return false;

I don't think we need a new attribute. CC_ATTR_MEM_ENCRYPT has to be
enough.

> @@ -134,6 +135,11 @@ __init void cc_set_vendor(enum cc_vendor v)
>         vendor = v;
>  }
>  
> +enum cc_vendor cc_get_vendor(void)
> +{
> +       return vendor;
> +}
> +
>  __init void cc_set_mask(u64 mask)
>  {
>         cc_mask = mask;
> 
> +++ b/arch/x86/coco/shmem.c
> @@ -0,0 +1,156 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Confidential Computing Shared Memory Allocator
> + *
> + * Copyright (C) 2022 Intel Corporation, Inc.
> + *
> + */
> +
> +#undef pr_fmt
> +#define pr_fmt(fmt)     "CC: " fmt
> +
> +#include <linux/export.h>
> +#include <linux/cma.h>
> +#include <linux/mm.h>
> +#include <linux/cc_platform.h>
> +#include <linux/set_memory.h>
> +#include <linux/platform_device.h>
> +#include <linux/dma-mapping.h>
> +
> +#include <asm/coco.h>
> +#include <asm/processor.h>
> +
> +#define CC_SHMEM_DRIVER                "cc-shmem"
> +
> +struct platform_device *shmem_pdev;
> +
> +struct shmem_priv
> +{
> +       int (*init)(struct platform_device *dev);
> +       void* (*alloc)(size_t size, gfp_t gfp, struct shmem_priv *priv);
> +       void (*free)(void *addr, size_t size, struct shmem_priv *priv);
> +       struct platform_device *pdev;
> +       void *data;
> +};
> +
> +void *cc_shmem_alloc(size_t size, gfp_t gfp)
> +{
> +       struct shmem_priv *priv;
> +
> +       if (!shmem_pdev)
> +               return NULL;
> +
> +       priv = platform_get_drvdata(shmem_pdev);
> +
> +       return priv->alloc(size, gfp, priv);
> +}
> +
> +void cc_shmem_free(void *addr, size_t size)
> +{
> +       struct shmem_priv *priv;
> +
> +       if (!shmem_pdev)
> +               return;
> +
> +       priv = platform_get_drvdata(shmem_pdev);
> +
> +       priv->free(addr, size, priv);
> +}
> +
> +static int intel_shmem_init(struct platform_device *pdev)
> +{
> +       struct shmem_priv *priv;
> +       dma_addr_t *handle;
> +
> +       handle = devm_kmalloc(&pdev->dev, sizeof(*handle), GFP_KERNEL);
> +       if (!handle)
> +               return -ENOMEM;
> +
> +       priv = platform_get_drvdata(pdev);
> +
> +       priv->data = handle;
> +
> +       return dma_set_coherent_mask(&pdev->dev, DMA_BIT_MASK(64));
> +}
> +
> +static void *intel_shmem_alloc(size_t size, gfp_t gfp, struct shmem_priv *priv)
> +{
> +       dma_addr_t *handle = (dma_addr_t *) priv->data;
> +
> +       return dma_alloc_coherent(&priv->pdev->dev, size, handle, gfp);
> +}
> +
> +static void intel_shmem_free(void *addr, size_t size, struct shmem_priv *priv)
> +{
> +       dma_addr_t *handle = (dma_addr_t *) priv->data;
> +
> +       return dma_free_coherent(&priv->pdev->dev, size, addr, *handle);
> +}
> +
> +static struct shmem_priv intel_shmem = {
> +       .init  = intel_shmem_init,
> +       .alloc = intel_shmem_alloc,
> +       .free  = intel_shmem_free,
> +};

Hm. What is Intel-specific here. Looks like a generic thing, no?

Maybe just drop all vendor stuff. CC_ATTR_MEM_ENCRYPT should be enough.


-- 
 Kirill A. Shutemov