On 1/12/23 09:28, Laszlo Ersek wrote:
> In QEMU v5.1.0, the CPU hotplug register block misbehaves: the negotiation
> protocol is (effectively) broken such that it suggests that switching from
> the legacy interface to the modern interface works, but in reality the
> switch never happens. The symptom has been witnessed when using TCG
> acceleration; KVM seems to mask the issue. The issue persists with the
> following (latest) stable QEMU releases: v5.2.0, v6.2.0, v7.2.0. Currently
> there is no stable release that addresses the problem.
>
> The QEMU bug confuses the Present and Possible counting in function
> PlatformMaxCpuCountInitialization(), in
> "OvmfPkg/Library/PlatformInitLib/Platform.c". OVMF ends up with Present=0
> Possible=1. This in turn further confuses MpInitLib in UefiCpuPkg (hence
> firmware-time multiprocessing will be broken). Worse, CPU hot(un)plug with
> SMI will be summarily broken in OvmfPkg/CpuHotplugSmm, which (considering
> the privilege level of SMM) is not that great.
>
> Detect the issue in PlatformMaxCpuCountInitialization(), and print an
> error message and *hang* if the issue is present.
>
> The problem was originally reported by Ard [0]. We analyzed it at [1] and
> [2]. A QEMU patch was sent at [3]; now merged as commit dab30fbef389
> ("acpi: cpuhp: fix guest-visible maximum access size to the legacy reg
> block", 2023-01-08), to be included in QEMU v8.0.0.
>
> [0] https://bugzilla.tianocore.org/show_bug.cgi?id=4234#c2
>
> [1] https://bugzilla.tianocore.org/show_bug.cgi?id=4234#c3
>
> [2] IO port write width clamping differs between TCG and KVM
>
> http://mid.mail-archive.com/[email protected]
> https://lists.gnu.org/archive/html/qemu-devel/2023-01/msg00199.html
>
> [3] acpi: cpuhp: fix guest-visible maximum access size to the legacy reg block
> http://mid.mail-archive.com/[email protected]
> https://lists.gnu.org/archive/html/qemu-devel/2023-01/msg00278.html
>
> NOTE: PlatformInitLib is used in the following platform DSCs:
>
> OvmfPkg/AmdSev/AmdSevX64.dsc
> OvmfPkg/CloudHv/CloudHvX64.dsc
> OvmfPkg/IntelTdx/IntelTdxX64.dsc
> OvmfPkg/Microvm/MicrovmX64.dsc
> OvmfPkg/OvmfPkgIa32.dsc
> OvmfPkg/OvmfPkgIa32X64.dsc
> OvmfPkg/OvmfPkgX64.dsc
>
> but I can only test this change with the last three platforms, running on
> QEMU.
>
> Test results:
>
> TCG QEMU OVMF result
> patched patched
> --- ------- ------- -------------------------------------------------
> 0 0 0 CPU counts OK (KVM masks the QEMU bug)
> 0 0 1 CPU counts OK (KVM masks the QEMU bug)
> 0 1 0 CPU counts OK (QEMU fix, but KVM masks the QEMU
> bug anyway)
> 0 1 1 CPU counts OK (QEMU fix, but KVM masks the QEMU
> bug anyway)
> 1 0 0 boot with broken CPU counts (original QEMU bug)
> 1 0 1 broken CPU count caught (boot hangs)
> 1 1 0 CPU counts OK (QEMU fix)
> 1 1 1 CPU counts OK (QEMU fix)
>
> Cc: Ard Biesheuvel <[email protected]>
> Cc: Brijesh Singh <[email protected]>
> Cc: Erdem Aktas <[email protected]>
> Cc: Gerd Hoffmann <[email protected]>
> Cc: James Bottomley <[email protected]>
> Cc: Jiewen Yao <[email protected]>
> Cc: Jordan Justen <[email protected]>
> Cc: Min Xu <[email protected]>
> Cc: Oliver Steffen <[email protected]>
> Cc: Sebastien Boeuf <[email protected]>
> Cc: Tom Lendacky <[email protected]>
> Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=4250
> Reviewed-by: Gerd Hoffmann <[email protected]>
> Signed-off-by: Laszlo Ersek <[email protected]>
> ---
>
> Notes:
> v2:
>
> - V1 was at
> <[email protected]">http://mid.mail-archive.com/[email protected]>.
>
> - Repo: <https://pagure.io/lersek/edk2.git>, branch:
> cpuhp-reg-catch-4250-v2
>
> - Remove KVM as a proposed workaround from the error message, because in
> the QEMU discussion, we had found that the KVM accelerator's behavior
> in QEMU (masking the problem) was not right, and that a fix for that
> had been in progress for quite some time.
>
> - Add the QEMU commit hash to the commit message, the code comment, and
> the error message.
>
> - Pick up Gerd's R-b; add Oliver to the Cc list.
>
> OvmfPkg/Library/PlatformInitLib/Platform.c | 35 ++++++++++++++++++++
> 1 file changed, 35 insertions(+)
>
> diff --git a/OvmfPkg/Library/PlatformInitLib/Platform.c
> b/OvmfPkg/Library/PlatformInitLib/Platform.c
> index 3e13c5d4b34f..13348afb4890 100644
> --- a/OvmfPkg/Library/PlatformInitLib/Platform.c
> +++ b/OvmfPkg/Library/PlatformInitLib/Platform.c
> @@ -541,6 +541,41 @@ PlatformMaxCpuCountInitialization (
> ASSERT (Selected == Possible || Selected == 0);
> } while (Selected > 0);
>
> + //
> + // Sanity check: we need at least 1 present CPU (CPU#0 is always
> present).
> + //
> + // The legacy-to-modern switching of the CPU hotplug register block got
> + // broken (for TCG) in QEMU v5.1.0. Refer to "IO port write width
> clamping
> + // differs between TCG and KVM" at
> + //
> <http://mid.mail-archive.com/[email protected]>
> + // or at
> + //
> <https://lists.gnu.org/archive/html/qemu-devel/2023-01/msg00199.html>.
> + //
> + // QEMU received the fix in commit dab30fbef389 ("acpi: cpuhp: fix
> + // guest-visible maximum access size to the legacy reg block",
> + // 2023-01-08), to be included in QEMU v8.0.0.
> + //
> + // If we're affected by this QEMU bug, then we must not continue: it
> + // confuses the multiprocessing in UefiCpuPkg/Library/MpInitLib, and
> + // breaks CPU hot(un)plug with SMI in OvmfPkg/CpuHotplugSmm.
> + //
> + if (Present == 0) {
> + DEBUG ((
> + DEBUG_ERROR,
> + "%a: Broken CPU hotplug register block: Present=%u Possible=%u.\n"
> + "%a: Update QEMU to v8, or to stable with dab30fbef389
> backported.\n"
> + "%a: Refer to "
> + "<https://bugzilla.tianocore.org/show_bug.cgi?id=4250>.\n",
> + __FUNCTION__,
> + Present,
> + Possible,
> + __FUNCTION__,
> + __FUNCTION__
> + ));
> + ASSERT (FALSE);
> + CpuDeadLoop ();
> + }
> +
> //
> // Sanity check: fw_cfg and the modern CPU hotplug interface should
> // return the same boot CPU count.
>
please do with this what you will
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