Re: [PATCHv2 23/29] x86/tdx: Add helper to convert memory between shared and private

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, Jan 24 2022 at 18:02, Kirill A. Shutemov wrote:
> Intel TDX protects guest memory from VMM access. Any memory that is
> required for communication with the VMM must be explicitly shared.
>
> It is a two-step process: the guest sets the shared bit in the page
> table entry and notifies VMM about the change. The notification happens
> using MapGPA hypercall.
>
> Conversion back to private memory requires clearing the shared bit,
> notifying VMM with MapGPA hypercall following with accepting the memory
> with AcceptPage hypercall.
>
> Provide a helper to do conversion between shared and private memory.
> It is going to be used by the following patch.

Strike that last sentence...

> --- a/arch/x86/kernel/tdx.c
> +++ b/arch/x86/kernel/tdx.c
> @@ -13,6 +13,10 @@
>  /* TDX module Call Leaf IDs */
>  #define TDX_GET_INFO			1
>  #define TDX_GET_VEINFO			3
> +#define TDX_ACCEPT_PAGE			6
> +
> +/* TDX hypercall Leaf IDs */
> +#define TDVMCALL_MAP_GPA		0x10001
>  
>  /* See Exit Qualification for I/O Instructions in VMX documentation */
>  #define VE_IS_IO_IN(exit_qual)		(((exit_qual) & 8) ? 1 : 0)
> @@ -97,6 +101,80 @@ static void tdx_get_info(void)
>  	td_info.attributes = out.rdx;
>  }
>  
> +static bool tdx_accept_page(phys_addr_t gpa, enum pg_level pg_level)
> +{
> +	/*
> +	 * Pass the page physical address to the TDX module to accept the
> +	 * pending, private page.
> +	 *
> +	 * Bits 2:0 if GPA encodes page size: 0 - 4K, 1 - 2M, 2 - 1G.
> +	 */
> +	switch (pg_level) {
> +	case PG_LEVEL_4K:
> +		break;
> +	case PG_LEVEL_2M:
> +		gpa |= 1;
> +		break;
> +	case PG_LEVEL_1G:
> +		gpa |= 2;
> +		break;
> +	default:
> +		return true;

Crack. boolean return true means success. Can we please keep this
convention straight throughout the code and not as you see fit?

This random choice of return code meanings is just a recipe for
disaster. Consistency matters.

> +	}
> +
> +	return __tdx_module_call(TDX_ACCEPT_PAGE, gpa, 0, 0, 0, NULL);
> +}
> +
> +/*
> + * Inform the VMM of the guest's intent for this physical page: shared with
> + * the VMM or private to the guest.  The VMM is expected to change its mapping
> + * of the page in response.
> + */
> +int tdx_hcall_request_gpa_type(phys_addr_t start, phys_addr_t end, bool enc)
> +{
> +	u64 ret;
> +
> +	if (end <= start)
> +		return -EINVAL;
> +
> +	if (!enc) {
> +		start |= tdx_shared_mask();
> +		end |= tdx_shared_mask();
> +	}
> +
> +	/*
> +	 * Notify the VMM about page mapping conversion. More info about ABI
> +	 * can be found in TDX Guest-Host-Communication Interface (GHCI),
> +	 * sec "TDG.VP.VMCALL<MapGPA>"
> +	 */
> +	ret = _tdx_hypercall(TDVMCALL_MAP_GPA, start, end - start, 0, 0, NULL);
> +
> +	if (ret)
> +		ret = -EIO;
> +
> +	if (ret || !enc)
> +		return ret;
> +
> +	/*
> +	 * For shared->private conversion, accept the page using
> +	 * TDX_ACCEPT_PAGE TDX module call.
> +	 */
> +	while (start < end) {
> +		/* Try 2M page accept first if possible */

Talking about consistency:

tdx_accept_page() implements 1G maps, but they are not required to be
handled here for some random reason, right?

> +		if (!(start & ~PMD_MASK) && end - start >= PMD_SIZE &&
> +		    !tdx_accept_page(start, PG_LEVEL_2M)) {
> +			start += PMD_SIZE;
> +			continue;
> +		}
> +
> +		if (tdx_accept_page(start, PG_LEVEL_4K))
> +			return -EIO;
> +		start += PAGE_SIZE;
> +	}

Thanks,

        tglx