89 lines
3.2 KiB
Diff
89 lines
3.2 KiB
Diff
From 6005efab3539eee16c131afe57f1b15237a4db77 Mon Sep 17 00:00:00 2001
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From: "Richard W.M. Jones" <rjones@redhat.com>
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Date: Thu, 12 May 2022 08:36:37 +0100
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Subject: [PATCH] lib: Disable 5-level page tables when using -cpu max
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In https://bugzilla.redhat.com/show_bug.cgi?id=2082806 we've been
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tracking an insidious qemu bug which intermittently prevents the
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libguestfs appliance from starting. The symptoms are that SeaBIOS
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starts and displays its messages, but the kernel isn't reached. We
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found that the kernel does in fact start, but when it tries to set up
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page tables and jump to protected mode it gets a triple fault which
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causes the emulated CPU in qemu to reset (qemu exits).
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This seems to only affect TCG (not KVM).
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Yesterday I found that this is caused by using -cpu max which enables
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the "la57" feature (5-level page tables[0]), and that we can make the
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problem go away using -cpu max,la57=off. Note that I still don't
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fully understand the qemu bug, so this is only a workaround.
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I chose to disable 5-level page tables for both TCG and KVM, partly to
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make the patch simpler, and partly because I guess it's not a feature
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(ie. 57 bit linear addresses) that is useful for the libguestfs
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appliance case, where we have limited physical memory and no need to
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run any programs with huge address spaces.
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I tested this by running both the direct & libvirt paths overnight. I
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expect that this patch will fail with old qemu/libvirt which doesn't
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understand the "la57" feature, but this is only intended as a
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temporary workaround.
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[0] Article about 5-level page tables as background:
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https://lwn.net/Articles/717293/
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Thanks: Laszlo Ersek
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Fixes: https://answers.launchpad.net/ubuntu/+source/libguestfs/+question/701625
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---
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lib/launch-direct.c | 15 +++++++++++++--
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lib/launch-libvirt.c | 7 +++++++
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2 files changed, 20 insertions(+), 2 deletions(-)
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diff --git a/lib/launch-direct.c b/lib/launch-direct.c
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index c07a8d78f..ff0eaeb62 100644
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--- a/lib/launch-direct.c
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+++ b/lib/launch-direct.c
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@@ -518,8 +518,19 @@ launch_direct (guestfs_h *g, void *datav, const char *arg)
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} end_list ();
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cpu_model = guestfs_int_get_cpu_model (has_kvm && !force_tcg);
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- if (cpu_model)
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- arg ("-cpu", cpu_model);
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+ if (cpu_model) {
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+#if defined(__x86_64__)
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+ /* Temporary workaround for RHBZ#2082806 */
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+ if (STREQ (cpu_model, "max")) {
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+ start_list ("-cpu") {
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+ append_list (cpu_model);
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+ append_list ("la57=off");
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+ } end_list ();
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+ }
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+ else
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+#endif
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+ arg ("-cpu", cpu_model);
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+ }
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if (g->smp > 1)
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arg_format ("-smp", "%d", g->smp);
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diff --git a/lib/launch-libvirt.c b/lib/launch-libvirt.c
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index 87da2f40e..03d69e027 100644
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--- a/lib/launch-libvirt.c
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+++ b/lib/launch-libvirt.c
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@@ -1185,6 +1185,13 @@ construct_libvirt_xml_cpu (guestfs_h *g,
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else if (STREQ (cpu_model, "max")) {
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/* https://bugzilla.redhat.com/show_bug.cgi?id=1935572#c11 */
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attribute ("mode", "maximum");
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+#if defined(__x86_64__)
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+ /* Temporary workaround for RHBZ#2082806 */
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+ start_element ("feature") {
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+ attribute ("policy", "disable");
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+ attribute ("name", "la57");
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+ } end_element ();
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+#endif
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}
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else
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single_element ("model", cpu_model);
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--
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2.31.1
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