On 3/27/2024 1:48 AM, Isaku Yamahata wrote:
On Tue, Mar 26, 2024 at 07:13:46PM +0800,
Chao Gao <chao.gao@xxxxxxxxx> wrote:
On Tue, Mar 26, 2024 at 10:42:36AM +0800, Edgecombe, Rick P wrote:
On Tue, 2024-03-26 at 10:32 +0800, Chao Gao wrote:
Something like this for "112/130 KVM: TDX: Handle TDX PV rdmsr/wrmsr hypercall"
Compile only tested at this point.
Seems reasonable to me. Does QEMU configure a special set of MSRs to filter for TDX currently?
No for TDX at the moment. We need to add such logic.
What if QEMU doesn't configure the set of MSRs to filter? In this case, KVM
still needs to handle the MSR accesses.
Do you see a problem for the kernel? I think if any issues are limited to only the guest, then we
should count on userspace to configure the msr list.
How can QEMU handle MTRR MSR accesses if KVM exits to QEMU? I am not sure if
QEMU needs to do a lot of work to virtualize MTRR.
The default kernel logic will to return error for
TDG.VP.VMCALL<RDMSR or WRMSR MTRR registers>.
Qemu can have mostly same in the current kernel logic.
rdmsr:
MTRRCAP: 0
MTRRDEFTYPE: MTRR_TYPE_WRBACK
wrmsr:
MTRRDEFTYPE: If write back, nop. Otherwise error.
If QEMU doesn't configure the msr filter list correctly, KVM has to handle
guest's MTRR MSR accesses. In my understanding, the suggestion is KVM zap
private memory mappings. But guests won't accept memory again because no one
currently requests guests to do this after writes to MTRR MSRs. In this case,
guests may access unaccepted memory, causing infinite EPT violation loop
(assume SEPT_VE_DISABLE is set). This won't impact other guests/workloads on
the host. But I think it would be better if we can avoid wasting CPU resource
on the useless EPT violation loop.
Qemu is expected to do it correctly. There are manyways for userspace to go
wrong. This isn't specific to MTRR MSR.
This seems incorrect. KVM shouldn't force userspace to filter some
specific MSRs. The semantic of MSR filter is userspace configures it on
its own will, not KVM requires to do so.
