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

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, 2021-09-10 at 17:10 +0200, Paolo Bonzini wrote:
> 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.

1: Why is it a problem that mmap() could fail?

2: Are you speaking about removing device node? If you have succesfully
   mapped /dev/sgx_vepc, that should not have much effect (file refcount).

/Jarkko