From b6a062c64f9639558a88f46edc3dd76b54b26bb5 Mon Sep 17 00:00:00 2001

From: Eduardo Habkost <ehabkost@redhat.com>

Date: Thu, 13 Dec 2018 15:51:59 +0000

Subject: [PATCH 1/5] x86: host-phys-bits-limit option

RH-Author: Eduardo Habkost <ehabkost@redhat.com>

Message-id: <20181213155200.20300-2-ehabkost@redhat.com>

Patchwork-id: 83479

O-Subject: [RHEL8/rhel qemu-kvm PATCH 1/2] x86: host-phys-bits-limit option

Bugzilla: 1598284

RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

RH-Acked-by: Pankaj Gupta <pagupta@redhat.com>

RH-Acked-by: Bandan Das <bsd@redhat.com>

RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Some downstream distributions of QEMU set host-phys-bits=on by

default.  This worked very well for most use cases, because

phys-bits really didn't have huge consequences. The only

difference was on the CPUID data seen by guests, and on the

handling of reserved bits.

This changed in KVM commit 855feb673640 ("KVM: MMU: Add 5 level

EPT & Shadow page table support").  Now choosing a large

phys-bits value for a VM has bigger impact: it will make KVM use

5-level EPT even when it's not really necessary.  This means

using the host phys-bits value may not be the best choice.

Management software could address this problem by manually

configuring phys-bits depending on the size of the VM and the

amount of MMIO address space required for hotplug.  But this is

not trivial to implement.

However, there's another workaround that would work for most

cases: keep using the host phys-bits value, but only if it's

smaller than 48.  This patch makes this possible by introducing a

new "-cpu" option: "host-phys-bits-limit".  Management software

or users can make sure they will always use 4-level EPT using:

"host-phys-bits=on,host-phys-bits-limit=48".

This behavior is still not enabled by default because QEMU

doesn't enable host-phys-bits=on by default.  But users,

management software, or downstream distributions may choose to

change their defaults using the new option.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

Message-Id: <20181211192527.13254-1-ehabkost@redhat.com>

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>

---

 target/i386/cpu.c | 5 +++++

 target/i386/cpu.h | 3 +++

 2 files changed, 8 insertions(+)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c

index a44912c..c37cd1e 100644

--- a/target/i386/cpu.c

+++ b/target/i386/cpu.c

@@ -4826,6 +4826,10 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)

             if (cpu->host_phys_bits) {

                 /* The user asked for us to use the host physical bits */

                 cpu->phys_bits = host_phys_bits;

+                if (cpu->host_phys_bits_limit &&

+                    cpu->phys_bits > cpu->host_phys_bits_limit) {

+                    cpu->phys_bits = cpu->host_phys_bits_limit;

+                }

             }

             /* Print a warning if the user set it to a value that's not the

@@ -5377,6 +5381,7 @@ static Property x86_cpu_properties[] = {

     DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),

     DEFINE_PROP_UINT32("phys-bits", X86CPU, phys_bits, 0),

     DEFINE_PROP_BOOL("host-phys-bits", X86CPU, host_phys_bits, false),

+    DEFINE_PROP_UINT8("host-phys-bits-limit", X86CPU, host_phys_bits_limit, 0),

     DEFINE_PROP_BOOL("fill-mtrr-mask", X86CPU, fill_mtrr_mask, true),

     DEFINE_PROP_UINT32("level", X86CPU, env.cpuid_level, UINT32_MAX),

     DEFINE_PROP_UINT32("xlevel", X86CPU, env.cpuid_xlevel, UINT32_MAX),

diff --git a/target/i386/cpu.h b/target/i386/cpu.h

index 4a3ef4b..58d5430 100644

--- a/target/i386/cpu.h

+++ b/target/i386/cpu.h

@@ -1418,6 +1418,9 @@ struct X86CPU {

     /* if true override the phys_bits value with a value read from the host */

     bool host_phys_bits;

+    /* if set, limit maximum value for phys_bits when host_phys_bits is true */

+    uint8_t host_phys_bits_limit;

+

     /* Stop SMI delivery for migration compatibility with old machines */

     bool kvm_no_smi_migration;

-- 

1.8.3.1