Re: [PATCH 25/25] KVM: x86: Add CPUID bits missing from KVM_GET_SUPPORTED_CPUID

Paolo Bonzini
2024-09-10T17:52:46Z

On 8/13/24 00:48, Rick Edgecombe wrote:
> Originally, the plan was to filter the directly configurable CPUID bits
> exposed by KVM_TDX_CAPABILITIES, and the final configured bit values
> provided by KVM_TDX_GET_CPUID. However, several issues were found with
> this. Both the filtering done with KVM_TDX_CAPABILITIES and
> KVM_TDX_GET_CPUID had the issue that the get_supported_cpuid() provided
> default values instead of supported masks for multi-bit fields (i.e. those
> encoding a multi-bit number).
>
> For KVM_TDX_CAPABILITIES, there was also the problem of bits that are
> actually supported by KVM, but missing from get_supported_cpuid() for one
> reason or another. These include X86_FEATURE_MWAIT, X86_FEATURE_HT and
> X86_FEATURE_TSC_DEADLINE_TIMER. This is currently worked around in QEMU by
> adjusting which features are expected. Some of these are going to be added
> to get_supported_cpuid(), and that is probably the right long term fix.

There are several cases here:

- MWAIT is hidden because it's hard to virtualize its C-state parameters

- HT is hidden because it depends on the topology, and cannot be added
blindly.

- TSC_DEADLINE_TIMER is queried with KVM_CHECK_EXTENSION for historical
reasons

There are basically two kinds of userspace:

- those that fetch KVM_GET_SUPPORED_CPUID and pass it blindly to
KVM_SET_CPUID2. These mostly work, though they may miss a feature or
three (e.g. the TSC deadline timer).

- those that know each bit and make an informed decision on what to
enable; for those, KVM_GET_SUPPORTED_CPUID is just guidance.

Because of this, KVM_GET_SUPPORTED_CPUID doesn't return bits that are
one; it returns a mix of:

- maximum supported values (e.g. CPUID[7,0].EAX)

- values from the host (e.g. FMS or model name)

- supported features

It's an awfully defined API but it is easier to use than it sounds (some
of the quirks are being documented in
Documentation/virt/kvm/x86/errata.rst and
Documentation/virt/kvm/x86/api.rst). The idea is that, if userspace
manages individual CPUID bits, it already knows what can be one anyway.

This is the kind of API that we need to present for TDX, even if the
details on how to get the supported CPUID are different. Not because
it's a great API, but rather because it's a known API.

The difference between this and KVM_GET_SUPPORTED_CPUID are small, but
the main one is X86_FEATURE_HYPERVISOR (I am not sure whether to make it
different with respect to X86_FEATURE_TSC_DEADLINE_TIMER; leaning
towards no).

We may also need a second ioctl specifically to return the fixed-1 bits.
Asking Xiaoyao for input with regard to what he'd like to have in QEMU.

> + entry = kvm_find_cpuid_entry2((*cpuid)->entries, (*cpuid)->nent, 0x0, 0);
> + if (WARN_ON(!entry))
> + goto err;
> + /* Fixup of maximum basic leaf */
> + entry->eax |= 0x000000FF;
> +
> + entry = kvm_find_cpuid_entry2((*cpuid)->entries, (*cpuid)->nent, 0x1, 0);
> + if (WARN_ON(!entry))
> + goto err;
> + /* Fixup of FMS */
> + entry->eax |= 0x0fff3fff;
> + /* Fixup of maximum logical processors per package */
> + entry->ebx |= 0x00ff0000;
> +

I see now why you could blindly AND things in patch 24.

However, the right mode of operation is still to pick manually which
bits to AND.

Paolo