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

Thomas Gleixner
2022-02-02T00:36:00Z

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