Re: [PATCH v8 2/3] x86/sgx: Introduce union with vepc_vaddr field for virtualization case

["Haitao Huang" <haitao.huang@linux.intel.com>]

Hi Zhiquan

On Tue, 13 Sep 2022 09:53:29 -0500, Zhiquan Li <zhiquan1.li@intel.com>  
wrote:

> When a page triggers a machine check, it only reports the PFN.  But in
> order to inject #MC into hypervisor, the virtual address is required.
> The 'encl_owner' field is useless in virtualization case, then
> repurpose it as 'vepc_vaddr' - the virtual address of the virtual EPC
> page for such case so that arch_memory_failure() can easily retrieve it.
>
> Introduce a union to prevent adding a new dedicated structure to
> track the virtual address of virtual EPC page.  And it can also prevent
> playing the casting games while using it.
>
> Add a new EPC page flag - SGX_EPC_PAGE_KVM_GUEST to interpret the
> meaning of the field.
>
> Co-developed-by: Cathy Zhang <cathy.zhang@intel.com>
> Signed-off-by: Cathy Zhang <cathy.zhang@intel.com>
> Signed-off-by: Zhiquan Li <zhiquan1.li@intel.com>
> Acked-by: Kai Huang <kai.huang@intel.com>
> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
> Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
>
> ---
> Changes since V7:
> - Add Acked-by from Jarkko.
>
> No changes since V6.
>
> Changes since V5:
> - To prevent casting the 'encl_owner' field, introduce a union with
>   another field - 'vepc_vaddr', sugguested by Dave Hansen.
> - Add Reviewed-by from Jarkko.
>   Link:  
> https://lore.kernel.org/linux-sgx/Yrf27fugD7lkyaek@kernel.org/T/#m379d00fc7f1d43726a42b3884637532061a8c0d1
>
> Changes since V4:
> - Add Co-developed-by and Signed-off-by from Cathy Zhang, as she had
>   fully discussed the flag name with Jarkko.
>   Link:  
> https://lore.kernel.org/all/df92395ade424401ac3c6322de568720@intel.com/
> - Add Acked-by from Kai Huang
>   Link:  
> https://lore.kernel.org/linux-sgx/0676cd4e-d94b-e904-81ae-ca1c05d37070@intel.com/T/#mccfb11df30698dbd060f2b6f06383cda7f154ef3
>
> Changes since V3:
> - Take the definition of EPC page flag SGX_EPC_PAGE_KVM_GUEST from
>   Cathy Zhang's third patch of SGX rebootless recovery patch set but
>   discard irrelevant portion, since it might need some time to
>   re-forge and these are two different features.
>   Link:  
> https://lore.kernel.org/linux-sgx/41704e5d4c03b49fcda12e695595211d950cfb08.camel@kernel.org/T/#m9782d23496cacecb7da07a67daa79f4b322ae170
>
> Changes since V2:
> - Remove struct sgx_vepc_page and relevant code.
> - Rework the patch suggested by Jarkko.
> - Remove new EPC page flag SGX_EPC_PAGE_IS_VEPC definition as it is
>   duplicated to SGX_EPC_PAGE_KVM_GUEST.
>   Link:  
> https://lore.kernel.org/linux-sgx/eb95b32ecf3d44a695610cf7f2816785@intel.com/T/#u
>
> Changes since V1:
> - Add documentation suggested by Jarkko.
> ---
>  arch/x86/kernel/cpu/sgx/main.c | 4 ++++
>  arch/x86/kernel/cpu/sgx/sgx.h  | 8 +++++++-
>  arch/x86/kernel/cpu/sgx/virt.c | 4 +++-
>  3 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/main.c  
> b/arch/x86/kernel/cpu/sgx/main.c
> index 1315c69a733e..b319bedcaf1e 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -549,6 +549,10 @@ int sgx_unmark_page_reclaimable(struct sgx_epc_page  
> *page)
>   * Finally, wake up ksgxd when the number of pages goes below the  
> watermark
>   * before returning back to the caller.
>   *
> + * When an EPC page is assigned to KVM guest, repurpose the  
> 'encl_owner' field
> + * as the virtual address of virtual EPC page, since it is useless in  
> such
> + * scenario, so 'owner' is assigned to 'vepc_vaddr'.
> + *
>   * Return:
>   *   an EPC page,
>   *   -errno on error
> diff --git a/arch/x86/kernel/cpu/sgx/sgx.h  
> b/arch/x86/kernel/cpu/sgx/sgx.h
> index 4d88abccd12e..d16a8baa28d4 100644
> --- a/arch/x86/kernel/cpu/sgx/sgx.h
> +++ b/arch/x86/kernel/cpu/sgx/sgx.h
> @@ -28,12 +28,18 @@
> /* Pages on free list */
>  #define SGX_EPC_PAGE_IS_FREE		BIT(1)
> +/* Pages allocated for KVM guest */
> +#define SGX_EPC_PAGE_KVM_GUEST		BIT(2)
> struct sgx_epc_page {
>  	unsigned int section;
>  	u16 flags;
>  	u16 poison;
> -	struct sgx_encl_page *encl_owner;
> +	union {
> +		struct sgx_encl_page *encl_owner;
> +		/* Use when SGX_EPC_PAGE_KVM_GUEST set in ->flags: */
> +		void __user *vepc_vaddr;
> +	};

Maybe it's just me missing some prior knowledge. It's not obvious to me  
why you don't need any guard accessing the encl_owner field in ksgxd  
thread. Is it because all vepc pages are never put in the active list and  
encl_owner would never be null for all pages in that list?
Regardless, could you add a few sentence here to to make the rule explicit?

Thanks
Haitao