Re: [Qemu-devel] [PATCH 1/2] qemu-kvm/cpuid: fix a emulation of guest physical address space

[Hao, Xudong]

> -----Original Message-----
> From: Jan Kiszka [mailto:address@hidden
> Sent: Saturday, November 03, 2012 6:55 PM
> To: Hao, Xudong
> Cc: address@hidden; address@hidden; address@hidden
> Subject: Re: [PATCH 1/2] qemu-kvm/cpuid: fix a emulation of guest physical
> address space
> 
> On 2012-11-02 06:38, Xudong Hao wrote:
> > For 64 bit processor, emulate 40 bits physical address if the host physical
> > address space >= 40bits, else guest physical is same as host.
> >
> > Signed-off-by: Xudong Hao <address@hidden>
> > ---
> >  target-i386/cpu.c |    5 ++++-
> >  1 files changed, 4 insertions(+), 1 deletions(-)
> >
> > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > index 423e009..3a78881 100644
> > --- a/target-i386/cpu.c
> > +++ b/target-i386/cpu.c
> > @@ -1584,7 +1584,10 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t
> index, uint32_t count,
> >          if (env->cpuid_ext2_features & CPUID_EXT2_LM) {
> >              /* 64 bit processor */
> >  /* XXX: The physical address space is limited to 42 bits in exec.c. */
> > -            *eax = 0x00003028;     /* 48 bits virtual, 40 bits physical */
> > +/* XXX: 40 bits physical if host physical address space >= 40 bits */
> > +            uint32_t a, b, c, d;
> > +            host_cpuid(0x80000008, 0, &a, &b, &c, &d);
> > +            *eax = a < 0x00003028 ? a : 0x00003028;
> 
> This variation will not only affect -cpu host, right? That can create
> problems when migrating between hosts with different address widths, and
> then we will need some control knob to adjust what it reported to the guest.
> 

Oh, I did not consider migrating to different platform(addr widths).
But I think the fixed value 40 bits may cause problem: in VT-d case, when a 
host support GAW < 40 bits, and qemu emulate 40 bits guest physical address 
space, will bring bug on:

drivers/iommu/intel-iommu.c
static struct dma_pte *pfn_to_dma_pte(struct dmar_domain *domain,
                      unsigned long pfn, int target_level)
{
    int addr_width = agaw_to_width(domain->agaw) - VTD_PAGE_SHIFT;
...
    BUG_ON(!domain->pgd);
    BUG_ON(addr_width < BITS_PER_LONG && pfn >> addr_width);

> Jan
> 
> >          } else {
> >              if (env->cpuid_features & CPUID_PSE36)
> >                  *eax = 0x00000024; /* 36 bits physical */
> >
>