Re: question on vTPM interface in coconut-svsm

[Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>]

On 28/03/2024 14:54, James Bottomley wrote:
> On Thu, 2024-03-28 at 14:41 +0100, Jeremi Piotrowski wrote:
>> On 28/03/2024 13:33, James Bottomley wrote:
>>> On Thu, 2024-03-28 at 13:22 +0100, Jeremi Piotrowski wrote:
>>> [...]
>>>> Azure ships the configuration described above for SEV-SNP (and
>>>> TDX).
>>>> The TPM is implemented in an "SVSM"(paravisor), exposed through
>>>> TPM
>>>> CRB MMIO. The kernel has a callback informing ioremap which MMIO
>>>> addresses should be considered shared/private [1]. This is the
>>>> Hyper-
>>>> v implementation of that callback: [2].
>>>>
>>>> So it can work if you detect it like this:
>>>>
>>>> if (SEV_SNP_GUEST && SVSM_PRESENT && SVSM_PROVIDES_VTPM)
>>>>    // vtpm should be mapped private
>>>
>>> Well, yes, it's pretty much identical to the detection mechanism
>>> used
>>> to activate the platform TPM driver:
>>>
>>> https://lore.kernel.org/all/83bcfc398d885f9e42d5aae42359fe02ab12d306.camel@linux.ibm.com/
>>>
>>> The SVSM_PROVIDES_VTPM is actually a dynamic probe to find the vTPM
>>> protocol inside the SVSM.
>>>
>>> So what's the mechanism hyper-v uses to start a CRB command?
>>>
>>> James
>>>
>>
>> Do you mean start method? The VTPM uses ACPI_TPM2_COMMAND_BUFFER, the
>> guest sees a TPM2 ACPI table, maps the control address as private and
>> then the tpm_crb driver just works.
> 
> Not without help.  The usual method of starting a CRB command is to
> write the command buffer and length into the CRB registers and then set
> the start bit (a real CRB device monitors the control area).  Since
> they're all in the same page, you can emulate what a real device does
> by unmapping this area in the guest kernel and getting the SVSM to
> intercept the writes, in which case you see a lot to fix up and quite a
> number of VMEXITs per command, or you can ignore this region and use
> some type of ASL start mechanism instead, which means only one VMEXIT.
> What does hyper-v do?
> 
> James

Now I get it, thanks. It's based on intercepting writes.

Jeremi