Re: [RFC PATCH 15/28] KVM: arm64: Handle realm MMIO emulation

Steven Price
2023-03-10T15:54:37Z

On 06/03/2023 15:37, Zhi Wang wrote:
> On Fri, 27 Jan 2023 11:29:19 +0000
> Steven Price <steven.price@arm.com> wrote:
>
>> MMIO emulation for a realm cannot be done directly with the VM's
>> registers as they are protected from the host. However the RMM interface
>> provides a structure member for providing the read/written value and
>
> More details would be better for helping the review. I can only see the
> emulated mmio value from the device model (kvmtool or kvm_io_bus) is put into
> the GPRS[0] of the RecEntry object. But the rest of the flow is missing.

The commit message is out of date (sorry about that). A previous version
of the spec had a dedicated member for the read/write value, but this
was changed to just use GPRS[0] as you've spotted. I'll update the text.

> I guess RMM copies the value in the RecEntry.GPRS[0] to the target GPR in the
> guest context in RMI_REC_ENTER when seeing RMI_EMULATED_MMIO. This is for
> the guest MMIO read path.

Yes, when entering the guest after an (emulatable) read data abort the
value in GPRS[0] is loaded from the RecEntry structure into the
appropriate register for the guest.

> How about the MMIO write path? I don't see where the RecExit.GPRS[0] is loaded
> to a varible and returned to the userspace.

The RMM will populate GPRS[0] with the written value in this case (even
if another register was actually used in the instruction). We then
transfer that to the usual VCPU structure so that the normal fault
handling logic works.

>> we can transfer this to the appropriate VCPU's register entry and then
>> depend on the generic MMIO handling code in KVM.
>>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> ---
>> arch/arm64/kvm/mmio.c | 7 +++++++
>> 1 file changed, 7 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
>> index 3dd38a151d2a..c4879fa3a8d3 100644
>> --- a/arch/arm64/kvm/mmio.c
>> +++ b/arch/arm64/kvm/mmio.c
>> @@ -6,6 +6,7 @@
>>
>> #include <linux/kvm_host.h>
>> #include <asm/kvm_emulate.h>
>> +#include <asm/rmi_smc.h>
>> #include <trace/events/kvm.h>
>>
>> #include "trace.h"
>> @@ -109,6 +110,9 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu)
>> &data);
>> data = vcpu_data_host_to_guest(vcpu, data, len);
>> vcpu_set_reg(vcpu, kvm_vcpu_dabt_get_rd(vcpu), data);
>> +
>> + if (vcpu_is_rec(vcpu))
>> + vcpu->arch.rec.run->entry.gprs[0] = data;
>
> I think the guest context is maintained by RMM (while KVM can only touch
> Rec{Entry, Exit} object) so that guest context in the legacy VHE mode is
> unused.
>
> If yes, I guess here is should be:
>
> if (unlikely(vcpu_is_rec(vcpu)))
> vcpu->arch.rec.run->entry.gprs[0] = data;
> else
> vcpu_set_reg(vcpu, kvm_vcpu_dabt_get_rd(vcpu), data);

Correct. Although there's no harm in updating with vcpu_set_reg(). But
I'll make the change because it's clearer.

>> }
>>
>> /*
>> @@ -179,6 +183,9 @@ int io_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa)
>> run->mmio.len = len;
>> vcpu->mmio_needed = 1;
>>
>> + if (vcpu_is_rec(vcpu))
>> + vcpu->arch.rec.run->entry.flags |= RMI_EMULATED_MMIO;
>> +
>
> Wouldn't it be better to set this in the kvm_handle_mmio_return where the MMIO
> read emulation has been surely successful?

Yes, that makes sense - I'll move this.

Thanks,

Steve

>> if (!ret) {
>> /* We handled the access successfully in the kernel. */
>> if (!is_write)
>