From 714c0396845fe74f8ccb9a72a7f0ab6753248e6e Mon Sep 17 00:00:00 2001

From: Eduardo Habkost <ehabkost@redhat.com>

Date: Thu, 11 Apr 2019 21:48:45 +0200

Subject: [PATCH 159/163] x86: host-phys-bits-limit option

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

Message-id: <20190411214846.8816-2-ehabkost@redhat.com>

Patchwork-id: 85608

O-Subject: [RHEL-7.7 qemu-kvm-rhev PATCH 1/2] x86: host-phys-bits-limit option

Bugzilla: 1691519

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

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

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

[ehabkost: removed test code while some issues are addressed]

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

(cherry picked from commit 258fe08bd341d2e230676228307294e41f33002c)

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

Signed-off-by: Miroslav Rezanina <mrezanin@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 2f6d5f4..c5156c8 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 75cf5ed..392065d 100644

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

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

@@ -1419,6 +1419,9 @@ 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