Re: [PATCH v4 09/15] RISC-V: KVM: Add perf sampling support for guests

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On 4/5/24 05:05, Andrew Jones wrote:
On Tue, Apr 02, 2024 at 01:33:10AM -0700, Atish Patra wrote:
but it should be possible for the VMM to disable this extension in the
guest. We just need to change all the checks in KVM of the host's ISA
for RISCV_ISA_EXT_SSCOFPMF to checking the guest's ISA instead. Maybe
it's not worth it, though, if the guest PMU isn't useful without overflow.
But, sometimes it's nice to be able to disable stuff for debug and

As per my understanding, kvm_riscv_vcpu_isa_disable_allowed only returns
true for those extensions which can be disabled architecturally.

I think kvm_riscv_vcpu_isa_disable_allowed can return true for any
extensions that KVM can guarantee won't be exposed in any way to the
guest. Extensions that cannot be disabled architecturally must return
false, since their instructions will still be present in the guest, even
if KVM doesn't want to expose them, but extensions which KVM emulates
can return true because KVM can choose not to emulate them. IIUC, sscofpmf
falls in this latter category.

hmm. The Sscofpmf is dependent on interrupt filtering via hvien and SBI PMU. So you are suggesting to toggle off the CSR_HVIEN bit for overflow interrupt or do more granular disabling for privilege mode filtering in SBI PMU as well.

Beyond that we can't disable SBI PMU. Is that okay ? A guest can still cause counter overflow and interrupt the host. However, the guest won't get any interrupt as hvien is not set.

It can also still filter the events as that is tied with SBI PMU.

We can put more granular checks in SBI pmu but I am just wondering if it provides anything additional beyond just disabling the sscofpmf in device tree.

VMM can still disable any extension by not adding to the device tree.
In fact, that's how kvmtool can disable sstc or sscofpmf with
--disable-<isa-ext command>.

The warning is bit confused though.

For example: if you run kvmtool with --disable-sstc

"Warning: Failed to disable sstc ISA exension"

I think Sstc should allow disabling since it has a corresponding henvcfg
bit which KVM could not set in order to force accesses to the Sstc CSRs
to raise ILL exceptions. So, let's put Sstc aside, since it's not a good

Agreed. I will send a separate patch for that.

example. An extension like Zihintpause, OTOH, cannot be disabled since
the 'pause' instruction will be present even if KVM does not put
Zihintpause in the guest's ISA string. If a kvmtool user uses
--disable-zihintpause, then I think this warning about failing to disable
the extension is appropriate.

But sstc is disabled: Here is the cpuinfo output.
# cat /proc/cpuinfo
processor       : 0
hart            : 0
isa             : rv64imafdc_zicbom_zicboz_zicntr_zicsr_zifencei_zihintntl_zihintpause_zihpm_zfa_zba_zbb_zbc_zbs_smstateen_sscofpmf
mmu             : sv57
mvendorid       : 0x0
marchid         : 0x0
mimpid          : 0x0
hart isa        : rv64imafdc_zicbom_zicboz_zicntr_zicsr_zifencei_zihintntl_zihintpause_zihpm_zfa_zba_zbb_zbc_zbs_smstateen_sscofpmf

Removing from the ISA string is the best we can do in cases like
Zihintpause, and is likely good enough for well-behaved guests, but the
VMM's warning to the user is good for these cases too, since not all
guests are well-behaved.


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