linux-kernel - Re: [PATCHv3.1 2.1/2] x86/coco: Add API to handle encryption mask

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Date:   Mon, 21 Feb 2022 12:31:40 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>, bp@...en8.de
Cc:     aarcange@...hat.com, ak@...ux.intel.com, dan.j.williams@...el.com,
        dave.hansen@...el.com, david@...hat.com, hpa@...or.com,
        jgross@...e.com, jmattson@...gle.com, joro@...tes.org,
        jpoimboe@...hat.com, kirill.shutemov@...ux.intel.com,
        knsathya@...nel.org, linux-kernel@...r.kernel.org, luto@...nel.org,
        mingo@...hat.com, pbonzini@...hat.com, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, sdeep@...are.com,
        seanjc@...gle.com, tglx@...utronix.de, tony.luck@...el.com,
        vkuznets@...hat.com, wanpengli@...cent.com, x86@...nel.org
Subject: Re: [PATCHv3.1 2.1/2] x86/coco: Add API to handle encryption mask

On 2/18/22 18:13, Kirill A. Shutemov wrote:
> AMD SME/SEV uses a bit in the page table entries to indicate that the
> page is encrypted and not accessible to the VMM.
> 
> TDX uses a similar approach, but the polarity of the mask is opposite to
> AMD: if the bit is set the page is accessible to VMM.
> 
> Provide vendor-neutral API to deal with the mask:
> 
>    - cc_mkenc() and cc_mkdec() modify given address to make it
>      encrypted/decrypted. It can be applied to phys_addr_t, pgprotval_t
>      or page table entry value.
> 
>    - cc_get_mask() returns encryption or decrypthion mask. It is useful
>      for set_memory_encrypted() and set_memory_decrypted()
>      implementation.
> 
> The implementation will be extended to cover TDX.
> 
> pgprot_decrypted() is used by drivers (i915, virtio_gpu, vfio).
> cc_mkdec() called by pgprot_decrypted(). Export cc_mkdec().
> 
> HyperV doesn't use bits in page table entries, so the mask is 0 for both
> encrypthion and decrypthion.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

> @@ -84,7 +85,39 @@ bool cc_platform_has(enum cc_attr attr)
>   }
>   EXPORT_SYMBOL_GPL(cc_platform_has);
>   
> -__init void cc_init(enum cc_vendor vendor)
> +u64 cc_get_mask(bool enc)

I was a bit confused by this name, expecting to always get the mask, but 
it is dependent on the input parameter. How about cc_get_operation_mask() 
or ... ? I'm struggling to come up with a good name, but plain 
cc_get_mask() is very confusing.

> +{
> +	switch (cc_vendor) {
> +	case CC_VENDOR_AMD:
> +		return enc ? cc_mask : 0;
> +	default:
> +		return 0;
> +	}
> +}
> +

> diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
> index eb7fbd85b77e..fa758247ab57 100644
> --- a/arch/x86/mm/mem_encrypt_identity.c
> +++ b/arch/x86/mm/mem_encrypt_identity.c
> @@ -603,5 +603,5 @@ void __init sme_enable(struct boot_params *bp)
>   out:
>   	physical_mask &= ~sme_me_mask;
>   	if (sme_me_mask)
> -		cc_init(CC_VENDOR_AMD);
> +		cc_init(CC_VENDOR_AMD, sme_me_mask);

Since you're adding the if statement, you can probably wrap the adjustment 
to physical_mask within the if, also (I guess in the previous patch). Not 
required, though.

Thanks,
Tom