Re: [PATCH kvmtool 06/21] hw/i8042: Refactor trap handler

Andre Przywara
2021-02-18T12:37:14Z

On Thu, 18 Feb 2021 11:17:58 +0000
Alexandru Elisei <alexandru.elisei@arm.com> wrote:

> Hi Andre,
>
> On 2/18/21 10:34 AM, Andre Przywara wrote:
> > On Thu, 11 Feb 2021 17:23:13 +0000
> > Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> >
> >> Hi Andre,
> >>
> >> On 12/10/20 2:28 PM, Andre Przywara wrote:
> >>> With the planned retirement of the special ioport emulation code, we
> >>> need to provide an emulation function compatible with the MMIO
> >>> prototype.
> >>>
> >>> Adjust the trap handler to use that new function, and provide shims to
> >>> implement the old ioport interface, for now.
> >>>
> >>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> >>> ---
> >>> hw/i8042.c | 68 +++++++++++++++++++++++++++---------------------------
> >>> 1 file changed, 34 insertions(+), 34 deletions(-)
> >>>
> >>> diff --git a/hw/i8042.c b/hw/i8042.c
> >>> index 36ee183f..eb1f9d28 100644
> >>> --- a/hw/i8042.c
> >>> +++ b/hw/i8042.c
> >>> @@ -292,52 +292,52 @@ static void kbd_reset(void)
> >>> };
> >>> }
> >>>
> >>> -/*
> >>> - * Called when the OS has written to one of the keyboard's ports (0x60 or 0x64)
> >>> - */
> >>> -static bool kbd_in(struct ioport *ioport, struct kvm_cpu *vcpu, u16 port, void *data, int size)
> >>> +static void kbd_io(struct kvm_cpu *vcpu, u64 addr, u8 *data, u32 len,
> >>> + u8 is_write, void *ptr)
> >>> {
> >>> - switch (port) {
> >>> - case I8042_COMMAND_REG: {
> >>> - u8 value = kbd_read_status();
> >>> - ioport__write8(data, value);
> >>> + u8 value;
> >>> +
> >>> + if (is_write)
> >>> + value = ioport__read8(data);
> >>> +
> >>> + switch (addr) {
> >>> + case I8042_COMMAND_REG:
> >>> + if (is_write)
> >>> + kbd_write_command(vcpu->kvm, value);
> >>> + else
> >>> + value = kbd_read_status();
> >>> break;
> >>> - }
> >>> - case I8042_DATA_REG: {
> >>> - u8 value = kbd_read_data();
> >>> - ioport__write8(data, value);
> >>> + case I8042_DATA_REG:
> >>> + if (is_write)
> >>> + kbd_write_data(value);
> >>> + else
> >>> + value = kbd_read_data();
> >>> break;
> >>> - }
> >>> - case I8042_PORT_B_REG: {
> >>> - ioport__write8(data, 0x20);
> >>> + case I8042_PORT_B_REG:
> >>> + if (!is_write)
> >>> + value = 0x20;
> >>> break;
> >>> - }
> >>> default:
> >>> - return false;
> >>> + return;
> >> Any particular reason for removing the false return value? I don't see it
> >> mentioned in the commit message. Otherwise this is identical to the two functions
> >> it replaces.
> > Because the MMIO handler prototype is void, in contrast to the PIO one.
> > Since on returning "false" we only seem to panic kvmtool, this is of
> > quite limited use, I'd say.
>
> Actually, in ioport.c::kvm__emulate_io(), if kvm->cfg.ioport_debug is true, it
> will print an error and then panic in kvm_cpu__start(); otherwise the error is
> silently ignored. serial.c is another device where an unknown register returns
> false. In rtc.c, the unknown register is ignored. cfi_flash.c prints debugging
> information. So I guess kvmtool implements all possible methods of handling an
> unknown register *at the same time*, so it's up to you how you want to handle it.

Well, the MMIO prototype we are going to use is void anyway, so it's
just one patch earlier that we get this new behaviour.
For handling MMIO errors:
- Hardware MMIO doesn't have a back channel: if the MMIO write triggers
some error condition, the device would need to deal with it (setting
internal error state, ignore, etc.). On some systems the device could
throw some kind of bus error or SError, but this is a rather drastic
measure, and is certainly not exercised by those ancient devices.
- Any kind of error reporting which can be triggered by a guest is
frowned upon: it could spam the console or some log file, and so
impact host operation. At the end an administrator can't do much about
it, anyway.
- Which leaves the only use to some kvmtool developer debugging some
device emulation or investigating weird guest behaviour. And in this
case we can more easily have a debug message *inside* the device
emulation code, can't we?

And since the MMIO handler prototype is void, we have no choice anyway,
at least not without another huge (and pointless) series to change
those user as well ;-)

Cheers,
Andre

> >>> }
> >>>
> >>> + if (!is_write)
> >>> + ioport__write8(data, value);
> >>> +}
> >>> +
> >>> +/*
> >>> + * Called when the OS has written to one of the keyboard's ports (0x60 or 0x64)
> >>> + */
> >>> +static bool kbd_in(struct ioport *ioport, struct kvm_cpu *vcpu, u16 port, void *data, int size)
> >>> +{
> >>> + kbd_io(vcpu, port, data, size, false, NULL);
> >> is_write is an u8, not a bool.
> > Right, will fix this.
> >
> >> I never could wrap my head around the ioport convention of "in" (read) and "out"
> >> (write). To be honest, changing is_write changed to an enum so it's crystal clear
> >> what is happening would help with that a lot, but I guess that's a separate patch.
> > "in" and "out" are the x86 assembly mnemonics, but it's indeed
> > confusing, because the device side has a different view (CPU "in" means
> > pushing data "out" of the device). I usually look at the code to see
> > what it's actually meant to do.
> > So yeah, I feel like a lot of those device emulations could use
> > some update. but that's indeed something for another day.
>
> Agreed.
>
> Thanks,
>
> Alex
>
> >
> > Cheers,
> > Andre
> >
> >>> +
> >>> return true;
> >>> }
> >>>
> >>> static bool kbd_out(struct ioport *ioport, struct kvm_cpu *vcpu, u16 port, void *data, int size)
> >>> {
> >>> - switch (port) {
> >>> - case I8042_COMMAND_REG: {
> >>> - u8 value = ioport__read8(data);
> >>> - kbd_write_command(vcpu->kvm, value);
> >>> - break;
> >>> - }
> >>> - case I8042_DATA_REG: {
> >>> - u8 value = ioport__read8(data);
> >>> - kbd_write_data(value);
> >>> - break;
> >>> - }
> >>> - case I8042_PORT_B_REG: {
> >>> - break;
> >>> - }
> >>> - default:
> >>> - return false;
> >>> - }
> >>> + kbd_io(vcpu, port, data, size, true, NULL);
> >>>
> >>> return true;
> >>> }