Re: [PATCH v6 06/13] x86/hyperv: Change vTOM handling to use standard coco mechanisms

[Borislav Petkov]

On Wed, Mar 08, 2023 at 06:40:07PM -0800, Michael Kelley wrote:
> diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
> index 49b44f8..d1c3306 100644
> --- a/arch/x86/coco/core.c
> +++ b/arch/x86/coco/core.c
> @@ -88,8 +106,6 @@ bool cc_platform_has(enum cc_attr attr)
>  		return amd_cc_platform_has(attr);
>  	case CC_VENDOR_INTEL:
>  		return intel_cc_platform_has(attr);
> -	case CC_VENDOR_HYPERV:
> -		return hyperv_cc_platform_has(attr);
>  	default:
>  		return false;
>  	}
> @@ -103,11 +119,14 @@ u64 cc_mkenc(u64 val)
>  	 * encryption status of the page.
>  	 *
>  	 * - for AMD, bit *set* means the page is encrypted
> -	 * - for Intel *clear* means encrypted.
> +	 * - for AMD with vTOM and for Intel, *clear* means encrypted
>  	 */
>  	switch (vendor) {
>  	case CC_VENDOR_AMD:
> -		return val | cc_mask;
> +		if (sev_status & MSR_AMD64_SNP_VTOM)
> +			return val & ~cc_mask;

This is silly. It should simply be:

		if (sev_status & MSR_AMD64_SNP_VTOM)
			return val;


> +		else
> +			return val | cc_mask;
>  	case CC_VENDOR_INTEL:
>  		return val & ~cc_mask;
>  	default:
> @@ -120,7 +139,10 @@ u64 cc_mkdec(u64 val)
>  	/* See comment in cc_mkenc() */
>  	switch (vendor) {
>  	case CC_VENDOR_AMD:
> -		return val & ~cc_mask;
> +		if (sev_status & MSR_AMD64_SNP_VTOM)
> +			return val | cc_mask;

So if you set the C-bit, that doesn't make it decrypted on AMD. cc_mask
on VTOM is 0 so why even bother?

Same as the above.

> +		else
> +			return val & ~cc_mask;
>  	case CC_VENDOR_INTEL:
>  		return val | cc_mask;
>  	default:

...

> +void __init hv_vtom_init(void)
> +{
> +	/*
> +	 * By design, a VM using vTOM doesn't see the SEV setting,
> +	 * so SEV initialization is bypassed and sev_status isn't set.
> +	 * Set it here to indicate a vTOM VM.
> +	 */

This looks like a hack. The SEV status MSR cannot be intercepted so the
guest should see vTOM. How are you running vTOM without setting it even up?!

> +	sev_status = MSR_AMD64_SNP_VTOM;
> +	cc_set_vendor(CC_VENDOR_AMD);
> +	cc_set_mask(ms_hyperv.shared_gpa_boundary);
> +	physical_mask &= ms_hyperv.shared_gpa_boundary - 1;
> +
> +	x86_platform.hyper.is_private_mmio = hv_is_private_mmio;
> +	x86_platform.guest.enc_cache_flush_required = hv_vtom_cache_flush_required;
> +	x86_platform.guest.enc_tlb_flush_required = hv_vtom_tlb_flush_required;
> +	x86_platform.guest.enc_status_change_finish = hv_vtom_set_host_visibility;
> +}
> +
> +#endif /* CONFIG_AMD_MEM_ENCRYPT */
> +
>  /*
>   * hv_map_memory - map memory to extra space in the AMD SEV-SNP Isolation VM.
>   */

-- 
Regards/Gruss,
    Boris.

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