Re: [PATCH v3 25/75] x86/sev-es: Add support for handling IOIO exceptions

Joerg Roedel
2020-06-03T14:23:31Z

Hi Jean,

On Tue, May 19, 2020 at 11:20:55PM -0700, Sean Christopherson wrote:
> On Tue, Apr 28, 2020 at 05:16:35PM +0200, Joerg Roedel wrote:
> > + /*
> > + * For the string variants with rep prefix the amount of in/out
> > + * operations per #VC exception is limited so that the kernel
> > + * has a chance to take interrupts an re-schedule while the
> > + * instruction is emulated.
>
> Doesn't this also suppress single-step #DBs?

Yes it does.

>
> > + */
> > + io_bytes = (exit_info_1 >> 4) & 0x7;
> > + ghcb_count = sizeof(ghcb->shared_buffer) / io_bytes;
> > +
> > + op_count = (exit_info_1 & IOIO_REP) ? regs->cx : 1;
> > + exit_info_2 = min(op_count, ghcb_count);
> > + exit_bytes = exit_info_2 * io_bytes;
> > +
> > + es_base = insn_get_seg_base(ctxt->regs, INAT_SEG_REG_ES);
> > +
> > + if (!(exit_info_1 & IOIO_TYPE_IN)) {
> > + ret = vc_insn_string_read(ctxt,
> > + (void *)(es_base + regs->si),
>
> SEV(-ES) is 64-bit only, why bother with the es_base charade?

User-space can also cause IOIO #VC exceptions, and user-space can be
32-bit legacy code with segments, so es_base has to be taken into
account.

>
> > + ghcb->shared_buffer, io_bytes,
> > + exit_info_2, df);
>
> df handling is busted, it's aways non-zero. Same goes for the SI/DI
> adjustments below.

Right, this is fixed now.

> Batching the memory accesses and I/O accesses separately is technically
> wrong, e.g. a #DB on a memory access will result in bogus data being shown
> in the debugger. In practice it seems unlikely to matter, but I'm curious
> as to why string I/O is supported in the first place. I didn't think there
> was that much string I/O in the kernel?

True, #DBs won't be correct anymore. Currently debugging is not
supported in SEV-ES guests anyway, but if it is supported the #DB
exception would happen in the #VC handler and not on the original
instruction.

Regards,

Joerg