Re: Handling faults on new HLV/HSV instructions in Hypervisor Extension draft 0.6


John Hauser
 

Greg Favor wrote:
When one of the new HLV/HSV instructions faults, what virtualization and
privilege modes are recorded in mstatus.mpp/mpv, or in sstatus.spp/spv and
hstatus.spvp? Are they based on the actual modes from within which the
instruction executes (i.e. on HS or U, and V=0), or on the effective modes
used by the instruction as it executes (i.e. on spvp and V=1).
The actual virtualization and privilege modes, same as always.

Consider the analogy with memory accesses made in M mode when
mstatus.MPRV = 1. The document says that such memory accesses occur
"as though the current privilege mode were set to MPP". If such a
memory access causes a trap, mstatus.MPP gets set to 3, the actual mode
at the time of the trap, not the "as-though" mode. As far as I know,
there's never been a question about this for MPRV.

Likewise, HLV and HSV are defined as performing memory accesses "as
though V = 1". Sounds the same to me.

I also think tables 5.6 and 5.7 in section 5.7.2, "Trap Entry", are
reasonably unambiguous on this point. Since HLV and HSV aren't said to
actually change the current virtualization or privilege modes, I feel
it's evident they don't affect what's written to SPV and SPP on a trap.

If instead the "effective modes applied", as you put it, then note
that SRET would no longer be sufficient to resume from a trap caused by
HLV/HSV. (Nor would MRET, if the trap is taken in M mode.)

FWIW, there's another clue hidden in this comment in section 5.2.1,
"Hypervisor Status Register (hstatus)":

For memory faults, GVA is redundant with field SPV (the two bits
are set the same) except when the explicit memory access of an HLV,
HLVX, or HSV instruction causes a fault. In that case, SPV = 0 but
GVA = 1.

Note, it says SPV gets set to 0, not 1.

Assuming, for example, the trap is taken into HS-mode:

- If the actual modes apply, then hstatus.spvp remains unchanged and
sstatus.spp/spv are set to reflect the actual privilege mode and V=0. The
hypervisor would then presumably figure out from htinst what caused this
trap?
Yes. Bit GVA in hstatus might also be helpful.

(In certain cases would the hypervisor need to save the original
'spp/spv' values before doing any HLV/HSV instructions so that it can
restore them afterwards?)
It is generally the case, whenever nested traps might be taken in
HS mode, that the hypervisor may need to save sstatus and hstatus
before the nested trap could occur, and restore them afterward. That's
no different than when an operating system might trigger a nested
S-mode-handled trap (like a page fault) by a memory access executed
in S mode: the OS may need to save and restore sstatus around such
possibilities. The specific situation determines exactly what must be
saved and restored.

- John Hauser

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