Reference model conflicts for privileged read for execute ATS request

[zetalog]

Considering ATS=1, PRIV=1 (ATS privileged request), a read for execute request with the following settings:

1. PV=1, DC.PDTV=1, PC.ENS=1, PC.SUM=1
2. VS PTE U=1, XWR=3'b111

Reference model responds to this request as success with
perm_X=0, perm_W=0, perm_R=0.
Is that correct? Shouldn't it be the following:
perm_X=0, perm_W=1, perm_R=1

The related code is as follows:

    // * Write/AMO guest-page fault (cause = 23)
    // * PDT entry not valid (cause = 266)
    // * MSI PTE not valid (cause = 262)
    if ( (cause == 12) || (cause == 13) || (cause == 15) || (cause == 20) || (cause == 21) ||
         (cause == 23) || (cause == 266) || (cause == 262) ) {
        // Return response with R=W=0. The rest of the field are undefined. The reference
        // model sets them to 0.
        vs_pte.raw = g_pte.raw = 0;
        is_msi = 0;
        page_sz = PAGESIZE;
        goto step_20;

Where vs_pte.raw = g_pte.raw = 0 causes this conflict. The reference model always returns a success response against such a page fault occurring to an ATS request, what bothers us a lot is the granted permission fields.

The ambiguity we can see from the specification:
If the translation request has a PASID with "Privilege Mode Requested“ field set to 1, then the request targets privileged memory. If the U-bit that indicates if the page is accessible to user mode is 1 and the SUM bit in the ta field of the process-context is 0 then a Success response with R and W bits set to 0 is generated.
If the translation could be successfully completed but the requested permission are not present (Execute requested but no execute permission; no-write not requested and no write permission; no read permission) then a Success response is returned with the denied permission (R, W or X) set to 0 and the other permission bits set to the value determined from the page table. The X permission is granted only if the R permission is also granted. Execute-only translations are not compatible with PCIe ATS as PCIe requires read permission to be granted if the execute permission is granted.

[ved-rivos]

The SUM bit only applies to read and write; it is not applicable to requests for execution. For a request to execute, the PTE with U=1 is not valid since that PTE is only for Unprivileged accesses. Hence it is treated as not being able to complete the translation.