Re: [PATCH RESEND v4 1/4] target/i386: Fix conditional CONFIG_SYNDBG enablement

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Michael Tokarev <mjt@tls.msk.ru> writes:

> 17.09.2024 19:00, Vitaly Kuznetsov пишет:
>> Putting HYPERV_FEAT_SYNDBG entry under "#ifdef CONFIG_SYNDBG" in
>> 'kvm_hyperv_properties' array is wrong: as HYPERV_FEAT_SYNDBG is not
>> the highest feature number, the result is an empty (zeroed) entry in
>> the array (and not a skipped entry!). hyperv_feature_supported() is
>> designed to check that all CPUID bits are set but for a zeroed
>> feature in 'kvm_hyperv_properties' it returns 'true' so QEMU considers
>> HYPERV_FEAT_SYNDBG as always supported, regardless of whether KVM host
>> actually supports it.
>> 
>> To fix the issue, leave HYPERV_FEAT_SYNDBG's definition in
>> 'kvm_hyperv_properties' array, there's nothing wrong in having it defined
>> even when 'CONFIG_SYNDBG' is not set. Instead, put "hv-syndbg" CPU property
>> under '#ifdef CONFIG_SYNDBG' to alter the existing behavior when the flag
>> is silently skipped in !CONFIG_SYNDBG builds.
>> 
>> Leave an 'assert' sentinel in hyperv_feature_supported() making sure there
>> are no 'holes' or improperly defined features in 'kvm_hyperv_properties'.
>> 
>> Fixes: d8701185f40c ("hw: hyperv: Initial commit for Synthetic Debugging device")
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>
>> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
>> index ada581c5d6ea..4009fcfd6b29 100644
>> --- a/target/i386/kvm/kvm.c
>> +++ b/target/i386/kvm/kvm.c
> ...
>> @@ -3924,13 +3929,11 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
>>                   kvm_msr_entry_add(cpu, HV_X64_MSR_TSC_EMULATION_STATUS,
>>                                     env->msr_hv_tsc_emulation_status);
>>               }
>> -#ifdef CONFIG_SYNDBG
>>               if (hyperv_feat_enabled(cpu, HYPERV_FEAT_SYNDBG) &&
>>                   has_msr_hv_syndbg_options) {
>>                   kvm_msr_entry_add(cpu, HV_X64_MSR_SYNDBG_OPTIONS,
>>                                     hyperv_syndbg_query_options());
>>               }
>> -#endif
>
> This change broke a minimal build:

Sorry about that :-(

>
> $ ../configure --without-default-features --without-default-devices --target-list=x86_64-softmmu --enable-kvm
> ...
> FAILED: qemu-system-x86_64
> cc -m64 @qemu-system-x86_64.rsp
> /usr/bin/ld: libqemu-x86_64-softmmu.a.p/target_i386_kvm_kvm.c.o: in function `kvm_put_msrs':
> target/i386/kvm/kvm.c:4039:(.text+0x83ae): undefined reference to `hyperv_syndbg_query_options'
> collect2: error: ld returned 1 exit status
>
> Why this particular #ifdef has been removed?

The patch was on the list for over a year before it got accepted and I
completely forgot the details... Looking at it now and I don't believe
we need HV_X64_MSR_SYNDBG_OPTIONS at all when !CONFIG_SYNDBG so I guess
we can bring the ifdef back. Let me do some quick tests and I'll send a
patch.

-- 
Vitaly