Re: [PATCH v6] hw/misc/vmfwupdate: Introduce hypervisor fw-cfg interface support

["Jörg Rödel" <joro@8bytes.org>]

Hi Gerd,

On Thu, Mar 13, 2025 at 01:05:13PM +0100, Gerd Hoffman wrote:
> // regions_addr points to an array of this structure
> struct vmfwupdate_regions {
>     uint64_t size;
>     uint64_t src_addr;       // source address (before update)
>     uint64_t dst_addr;       // destination address (after update)
>     uint64_t flags;          // control bits
> };
> 
> // flags
> #define VMFWUPDATE_REGION_FLAG_COPY     // data must be copied
> #define VMFWUPDATE_REGION_FLAG_ZERO     // dest must be cleared
> #define VMFWUPDATE_REGION_FLAG_MEASURE  // data must be measured
> 
> (1)   is still not covered.
> (2+3) can be handled with FLAG_ZERO regions, with and without
>       FLAG_MEASURE.
> (4)   Alex already pointed that the cpuid page is special, guess we
>       need additional flags for those oages.

That looks better, when the host VMM guarantees the order in which it
translates these regions into VM setup calls, then it is a step
forward. Although there more things to keep in mind, like the guest
policy and SEV status parameters.

> Open question is what we do about IGVM.
> 
> One option would be the guest vmfwupdate tool loading and parsing igvm,
> preparing the region list, then invoke the update.  Problem is that some
> igvm feaures such as initial register state can not be easily supported
> that way.
> 
> We could also expect the hypervisor support igvm, so the guest can
> simply load the file into memory and pass address and size to the
> hypervisor.  Either as option, say via VMFWUPDATE_REGION_FLAG_IGVM, or
> mandatory, i.e. scratch the region list and use IGVM exclusively.

This is of course up to the QEMU maintainers to decide, but I want to
highlight that IGVM already solves all the problems mentioned above,
including setting multiple memory regions of different type, special
data pages (cpuid, secrets, id-blob, vmsa) and more. It defines the
order of setup calls the VMM has to invoke for the new context and also
works for multiple platforms like TDX, SNP, non-coco, and in the future
ARM as well.

Regards,

	Joerg