Re: [PATCH v6 11/11] hvf: arm: Implement -cpu host

[Alexander Graf <agraf@csgraf.de>]

On 28.01.21 17:55, Peter Maydell wrote:
> On Wed, 20 Jan 2021 at 22:44, Alexander Graf <agraf@csgraf.de> wrote:
>> Now that we have working system register sync, we push more target CPU
>> properties into the virtual machine. That might be useful in some
>> situations, but is not the typical case that users want.
>>
>> So let's add a -cpu host option that allows them to explicitly pass all
>> CPU capabilities of their host CPU into the guest.
>>
>> Signed-off-by: Alexander Graf <agraf@csgraf.de>
>> Acked-by: Roman Bolshakov <r.bolshakov@yadro.com>
>> ---
>>  include/sysemu/hvf.h |  2 ++
>>  target/arm/cpu.c     |  9 ++++++---
>>  target/arm/cpu.h     |  2 ++
>>  target/arm/hvf/hvf.c | 41 +++++++++++++++++++++++++++++++++++++++++
>>  target/arm/kvm_arm.h |  2 --
>>  5 files changed, 51 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/sysemu/hvf.h b/include/sysemu/hvf.h
>> index f893768df9..7eb61cf094 100644
>> --- a/include/sysemu/hvf.h
>> +++ b/include/sysemu/hvf.h
>> @@ -19,6 +19,8 @@
>>  #ifdef CONFIG_HVF
>>  uint32_t hvf_get_supported_cpuid(uint32_t func, uint32_t idx,
>>                                   int reg);
>> +struct ARMCPU;
> Yuck.
>

[...]


>> +    hv_vcpu_t fd;
>> +    hv_vcpu_exit_t *exit;
>> +    int i;
>> +
>> +    cpu->dtb_compatible = "arm,arm-v8";
>> +    cpu->env.features = (1ULL << ARM_FEATURE_V8) |
>> +                        (1ULL << ARM_FEATURE_NEON) |
>> +                        (1ULL << ARM_FEATURE_AARCH64) |
>> +                        (1ULL << ARM_FEATURE_PMU) |
>> +                        (1ULL << ARM_FEATURE_GENERIC_TIMER);
>> +
>> +    /* We set up a small vcpu to extract host registers */
>> +
>> +    assert_hvf_ok(hv_vcpu_create(&fd, &exit, NULL));
>> +    for (i = 0; i < ARRAY_SIZE(regs); i++) {
>> +        assert_hvf_ok(hv_vcpu_get_sys_reg(fd, regs[i].reg, regs[i].val));
>> +    }
>> +    assert_hvf_ok(hv_vcpu_get_sys_reg(fd, HV_SYS_REG_MIDR_EL1, &cpu->midr));
>> +    assert_hvf_ok(hv_vcpu_destroy(fd));
> Nicer to follow the KVM approach of only doing this once
> and caching the results in arm_host_cpu_features, so that
> for a many-cores VM you don't do it once per core.


I tend to disagree. This scales nicely with the number of cores you have
in the system and it's blazingly fast :). I would rather like to
optimize for simplicity here.

The rest of the comments I agree with and will address them in v7 :)


Alex