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

[Brijesh Singh]

Hi Michael,

On 09/13/2016 01:39 PM, Michael S. Tsirkin wrote:
> On Tue, Sep 13, 2016 at 10:50:06AM -0400, Brijesh Singh wrote:
> > In SEV-enabled guest dma should be performed on shared pages. Since
> > the SeaBIOS executes in non PAE mode and does not have access to C-bit
> > to create a shared page hence disable the dma operation when reading
> > from fw_cfg interface.
> >
> > Signed-off-by: Brijesh Singh <address@hidden>
>
> Why do we need new interfaces for this though? Just
> set dma_enabled=false on command line.

Thanks, i was not aware of dma_enabled=false command line option.
Could you please tell me how to set this command line option?

I see this in 'qemu --help'

-fw_cfg [name=]<name>,file=<file>
                 add named fw_cfg entry from file
-fw_cfg [name=]<name>,string=<str>
                 add named fw_cfg entry from string


> > ---
> >  hw/nvram/fw_cfg.c |    6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> > index 6a68e59..aca99e9 100644
> > --- a/hw/nvram/fw_cfg.c
> > +++ b/hw/nvram/fw_cfg.c
> > @@ -24,6 +24,7 @@
> >  #include "qemu/osdep.h"
> >  #include "hw/hw.h"
> >  #include "sysemu/sysemu.h"
> > +#include "sysemu/kvm.h"
> >  #include "sysemu/dma.h"
> >  #include "hw/boards.h"
> >  #include "hw/isa/isa.h"
> > @@ -1009,6 +1010,11 @@ static void fw_cfg_io_realize(DeviceState *dev, Error 
> > **errp)
> >      FWCfgIoState *s = FW_CFG_IO(dev);
> >      SysBusDevice *sbd = SYS_BUS_DEVICE(dev);
> >
> > +    /* disable dma on fw_cfg when SEV is enabled */
> > +    if (kvm_sev_enabled()) {
> > +        qdev_prop_set_bit(dev, "dma_enabled", false);
> > +    }
> > +
> >      /* when using port i/o, the 8-bit data register ALWAYS overlaps
> >       * with half of the 16-bit control register. Hence, the total size
> >       * of the i/o region used is FW_CFG_CTL_SIZE */