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Re: [Qemu-devel] [PATCH RFC] acpi: add reset register to fadt

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH RFC] acpi: add reset register to fadt
Date: Tue, 31 Jan 2017 16:58:22 +0200

On Tue, Jan 31, 2017 at 03:31:46PM +0100, Phil Dennis-Jordan wrote:
> On 18 January 2017 at 18:19, Igor Mammedov <address@hidden> wrote:
> > On Wed, 18 Jan 2017 18:30:59 +0200
> > "Michael S. Tsirkin" <address@hidden> wrote:
> >
> >> On Wed, Jan 18, 2017 at 12:45:54PM +0100, Phil Dennis-Jordan wrote:
> > [...]
> >
> >> > I suspect more might be involved in enabling ACPI 2.0, and it should 
> >> > probably be an option so as to avoid regressions. I don't know what the 
> >> > best approach would be for this, so comments welcome. Should adding the 
> >> > reset register to the FADT also be configurable?
> >>
> >> I would say an option to make FADT use rev 3 format would be a good
> >> idea.
> >>
> >> I'd make it the default if XP survives.
> > if XP and legacy linux survive,
> > I'd skip adding option as probably there won't be any users,
> > in unlikely case such user surfaces we always can add option later
> > but we can't do it other way around (i.e. take it away).
> I have now finally solved the mystery of why my FADT patch has been
> going so disastrously wrong - I've now got working code, but I'd
> appreciate some guidance on the best way to structure a patch to
> minimise further back-and forth.

+ lersek

> The culprit turned out to be OVMF,
> specifically 2 bugs/shortcomings:
> 1. It completely gives up on parsing Qemu's ACPI tables if more than
> one "add pointer" linker command points to the same table. In this
> case, if you add a command for both the DSDT and X_DSDT fields of the
> FADT, it aborts completely and uses fallback tables. (The following
> InstallAcpiTable call fails if called twice with the same table type.)
> https://github.com/tianocore/edk2/blob/master/OvmfPkg/AcpiPlatformDxe/QemuFwCfgAcpi.c#L518
> 2. After applying all the linker commands, it goes and rewrites part
> of the FADT anyway. Specifically, it rewrites the DSDT and X_DSDT
> fields - and it always sets one of them to 0. Which one depends on
> whether the DSDT is above the 4G barrier:
> https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Universal/Acpi/AcpiTableDxe/AcpiTableProtocol.c#L650
> Both of these are easily fixed, and I will submit a corresponding patch to 
> EDK2.
> With that fixed, the rest of the FADT provided by Qemu is accepted by
> OVMF and the operating systems. On the Qemu side, it does mean we'll
> need to still retain the ACPI 1.0 tables for backwards compatibility.
> Q1: How should the option be structured and named? Should the FADT
> revision be selectable via a sub-option on -machine? Or as a
> standalone option? Something else?
> Q2: To avoid any more confusion, I'd appreciate
> confirmation/clarification on the X_ and non-X FADT fields in the case
> where 32-bit pointers suffice.
> Q2a: DSDT/X_DSDT: Both variants appear to be de-facto required.
> Q2c: X_PM1a_EVT_BLK, X_PM1a_CNT_BLK, X_PM_TMR_BLK: These all state
> "This is a required field" for both variants.
> Q2d: GPE0_BLK/X_GPE0_BLK: Both variants state "if this register block
> is not supported, this field contains zero." - I understand this to
> mean that when the register block IS supported and 32-bit, both
> variants must be filled.
> In other words, only X_FIRMWARE_CTRL stays zero in Qemu's x86 case.
> I'll come up with a revised patch in the next few days.
> Thanks for your help and patience so far, everyone!

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