Re: [Qemu-devel] [RFC PATCH v1 20/22] fw_cfg: sev: disable dma in real mode

[Paolo Bonzini]

On 14/09/2016 04:33, Michael S. Tsirkin wrote:
> Frankly I don't understand why do you need to mess with boot at all.
> Quoting the cover letter:
> 
>       SEV is designed to protect guest VMs from a benign but vulnerable
>       (i.e. not fully malicious) hypervisor. In particular, it reduces the
>       attack
>       surface of guest VMs and can prevent certain types of VM-escape bugs
>       (e.g. hypervisor read-anywhere) from being used to steal guest data.
> 
> it seems highly unlikely that any secret data is used during boot.
> So just let guest boot normally, and encrypt afterwards.
> 
> Even assuming there are some guests that have secret data during boot,
> I would first upstream the main part of the feature for normal guests,
> then weight the extra security if any against the features and
> performance lost (like slower boot times).

If you can't trust boot, any encryption done afterwards is totally
pointless.

Paolo