On Tue, Dec 11, 2018 at 05:25:27PM -0200, Eduardo Habkost wrote:
> 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 <ehabk...@redhat.com>
Thanks, Eduardo. One question is, should we check host-phys-bits-limit
against 48? If not, how about we just say in the commit message, that
the suggested value of host-phys-bits-limit is no bigger than 48 to
ensure a 4-level EPT? :-)
B.R.
Yu
> ---
> target/i386/cpu.h | 3 ++
> target/i386/cpu.c | 5 ++
> tests/acceptance/x86-phys-bits.py | 81 +++++++++++++++++++++++++++++++
> 3 files changed, 89 insertions(+)
> create mode 100644 tests/acceptance/x86-phys-bits.py
>
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 9c52d0cbeb..a545b77c2c 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -1457,6 +1457,9 @@ 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;
>
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index f81d35e1f9..df200754a2 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -5152,6 +5152,10 @@ 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
> @@ -5739,6 +5743,7 @@ 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/tests/acceptance/x86-phys-bits.py
> b/tests/acceptance/x86-phys-bits.py
> new file mode 100644
> index 0000000000..ae85d7e4e7
> --- /dev/null
> +++ b/tests/acceptance/x86-phys-bits.py
> @@ -0,0 +1,81 @@
> +# Test for host-phys-bits-limit option
> +#
> +# Copyright (c) 2018 Red Hat, Inc.
> +#
> +# Author:
> +# Eduardo Habkost <ehabk...@redhat.com>
> +#
> +# This work is licensed under the terms of the GNU GPL, version 2 or
> +# later. See the COPYING file in the top-level directory.
> +import re
> +
> +from avocado_qemu import Test
> +
> +class HostPhysbits(Test):
> + """
> + Check if `host-phys-bits` and `host-phys-bits` options work.
> +
> + :avocado: enable
> + :avocado: tags=x86_64
> + """
> +
> + def cpu_qom_get(self, prop):
> + qom_path = self.vm.command('query-cpus')[0]['qom_path']
> + return self.vm.command('qom-get', path=qom_path, property=prop)
> +
> + def cpu_phys_bits(self):
> + return self.cpu_qom_get('phys-bits')
> +
> + def host_phys_bits(self):
> + cpuinfo = open('/proc/cpuinfo', 'rb').read()
> + m = re.search(b'([0-9]+) bits physical', cpuinfo)
> + if m is None:
> + self.cancel("Couldn't read phys-bits from /proc/cpuinfo")
> + return int(m.group(1))
> +
> + def setUp(self):
> + super(HostPhysbits, self).setUp()
> + self.vm.add_args('-S', '-machine', 'accel=kvm:tcg')
> + self.vm.launch()
> + if not self.vm.command('query-kvm')['enabled']:
> + self.cancel("Test case requires KVM")
> + self.vm.shutdown()
> +
> +
> + def test_no_host_phys_bits(self):
> + # default should be phys-bits=40 if host-phys-bits=off
> + self.vm.add_args('-cpu', 'qemu64,host-phys-bits=off')
> + self.vm.launch()
> + self.assertEqual(self.cpu_phys_bits(), 40)
> +
> + def test_manual_phys_bits(self):
> + self.vm.add_args('-cpu', 'qemu64,host-phys-bits=off,phys-bits=35')
> + self.vm.launch()
> + self.assertEqual(self.cpu_phys_bits(), 35)
> +
> + def test_host_phys_bits(self):
> + host_phys_bits = self.host_phys_bits()
> + self.vm.add_args('-cpu', 'qemu64,host-phys-bits=on')
> + self.vm.launch()
> + self.assertEqual(self.cpu_phys_bits(), host_phys_bits)
> +
> + def test_host_phys_bits_limit_high(self):
> + hbits = self.host_phys_bits()
> + self.vm.add_args('-cpu', 'qemu64,host-phys-bits=on,'
> + 'host-phys-bits-limit=%d' % (hbits + 1))
> + self.vm.launch()
> + self.assertEqual(self.cpu_phys_bits(), hbits)
> +
> + def test_host_phys_bits_limit_equal(self):
> + hbits = self.host_phys_bits()
> + self.vm.add_args('-cpu', 'qemu64,host-phys-bits=on,'
> + 'host-phys-bits-limit=%d' % (hbits))
> + self.vm.launch()
> + self.assertEqual(self.cpu_phys_bits(), hbits)
> +
> + def test_host_phys_bits_limit_low(self):
> + hbits = self.host_phys_bits()
> + self.vm.add_args('-cpu', 'qemu64,host-phys-bits=on,'
> + 'host-phys-bits-limit=%d' % (hbits - 1))
> + self.vm.launch()
> + self.assertEqual(self.cpu_phys_bits(), hbits - 1)
> --
> 2.18.0.rc1.1.g3f1ff2140
>
>
