Re: [Qemu-devel] [PATCH] x86: host-phys-bits-limit option

[Wainer dos Santos Moschetta]

On 12/11/2018 05:25 PM, 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 <address@hidden>
> ---
>   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),

The changes to introduce the host-phys-bits-limit option looks good to 
me, although I haven't much knowledge on this area anyway...

> 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

PEP8 does not have a convention for Python files naming, although it 
suggests the use of underscore on module and package names. We already 
have the tests/acceptance/boot_linux_console.py with underscore, so I 
think we should make it a convention.


> @@ -0,0 +1,81 @@
> +# Test for host-phys-bits-limit option
> +#
> +# Copyright (c) 2018 Red Hat, Inc.
> +#
> +# Author:
> +#  Eduardo Habkost <address@hidden>
> +#
> +# 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.

Did you mean "`host-phys-bits` and `host-phys-bits-limit`?

> +
> +    :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)

pylint says:

"tests/acceptance/x86-phys-bits.py:31:8: C0103: Variable name "m" 
doesn't conform to snake_case naming style (invalid-name)"

It also warns about "Missing method docstring" for each test method. I 
don't know whether we should convention that every method must have a 
docstring explaining the test, or just ignore that warn.

> +        if m is None:
> +            self.cancel("Couldn't read phys-bits from /proc/cpuinfo")

I suggest something like 'bits physical size' instead of 'phys-bits'.

> +        return int(m.group(1))
> +
> +    def setUp(self):
> +        super(HostPhysbits, self).setUp()
> +        self.vm.add_args('-S', '-machine', 'accel=kvm:tcg')

I'm curious to know why you freeze the CPU at start (-S).

> +        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

Perhaps put that comment in a docstring block.

> +        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)

Suppose an user sets both host-phys-bits and phys-bits by mistake. What 
is the expected outcome? If this case is relevant...below test case 
fails (phys-bits is set to host's):

    def test_manual_phys_bits_mistake(self):
        """
        By mistake set host-phys-bits=on and phys-bits
        """
        self.vm.add_args('-cpu', 'qemu64,host-phys-bits=on,phys-bits=35')
        self.vm.launch()
        self.assertEqual(self.cpu_phys_bits(), 35)

- Wainer