Re: [PATCH v4 22/33] hostmem-epc: Add the reset interface for EPC backend reset

[Paolo Bonzini]

On 19/07/21 13:21, Yang Zhong wrote:
> +void sgx_memory_backend_reset(HostMemoryBackend *backend, int fd,
> +                              Error **errp)
> +{
> +    MemoryRegion *mr = &backend->mr;
> +
> +    mr->enabled = false;
> +
> +    /* destroy the old memory region if it exist */
> +    if (fd > 0 && mr->destructor) {
> +        mr->destructor(mr);
> +    }
> +
> +    sgx_epc_backend_memory_alloc(backend, errp);
> +}
> +

Jarkko, Sean, Kai,

this I think is problematic because it has a race window while 
/dev/sgx_vepc is closed and then reopened.  First, the vEPC space could 
be exhausted by somebody doing another mmap in the meanwhile.  Second, 
somebody might (for whatever reason) remove /dev/sgx_vepc while QEMU runs.

Yang explained to me (offlist) that this is needed because Windows fails 
to reboot without it.  We would need a way to ask Linux to reinitialize 
the vEPC, that doesn't involve munmap/mmap; this could be for example 
fallocate(FALLOC_FL_ZERO_RANGE).

What do you all think?

Paolo