Re: [Xen-devel] [PATCH v4 1/7] x86: provide executable fixmap facility

[Wei Liu <wl@xxxxxxx>]

On Wed, Jan 22, 2020 at 08:56:55PM +0000, Andrew Cooper wrote:
> On 22/01/2020 20:23, Wei Liu wrote:
> > diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
> > index 1cbf5acdfb..605d01f1dd 100644
> > --- a/xen/arch/x86/boot/x86_64.S
> > +++ b/xen/arch/x86/boot/x86_64.S
> > @@ -85,7 +85,15 @@ GLOBAL(l2_directmap)
> >   * 4k page.
> >   */
> 
> Adjust this comment as well?

I thought it was still accurate, so I didn't touch it.

Now it reads:

/*
 * L2 mapping the Xen text/data/bss region, constructed dynamically.
 * Executable fixmap region is hooked up statically.
 * Uses 1x * 4k page.
 */

Does this sound good to you?

> 
> > diff --git a/xen/include/asm-x86/config.h b/xen/include/asm-x86/config.h
> > index d0cfbb70a8..4fa56ea0a9 100644
> > --- a/xen/include/asm-x86/config.h
> > +++ b/xen/include/asm-x86/config.h
> > @@ -218,7 +218,7 @@ extern unsigned char boot_edid_info[128];
> >  /* Slot 261: high read-only compat machine-to-phys conversion table (1GB). 
> > */
> >  #define HIRO_COMPAT_MPT_VIRT_START RDWR_COMPAT_MPT_VIRT_END
> >  #define HIRO_COMPAT_MPT_VIRT_END (HIRO_COMPAT_MPT_VIRT_START + GB(1))
> > -/* Slot 261: xen text, static data and bss (1GB). */
> > +/* Slot 261: xen text, static data, bss and executable fixmap (1GB). */
> 
> And per-cpu stubs.  Might as well fix the comment while editing.

Ack.

> 
> >  #define XEN_VIRT_START          (HIRO_COMPAT_MPT_VIRT_END)
> >  #define XEN_VIRT_END            (XEN_VIRT_START + GB(1))
> >  
> > diff --git a/xen/include/asm-x86/fixmap.h b/xen/include/asm-x86/fixmap.h
> > index 9fb2f47946..c2a9d2b50a 100644
> > --- a/xen/include/asm-x86/fixmap.h
> > +++ b/xen/include/asm-x86/fixmap.h
> > @@ -15,6 +15,9 @@
> >  #include <asm/page.h>
> >  
> >  #define FIXADDR_TOP (VMAP_VIRT_END - PAGE_SIZE)
> > +#define FIXADDR_X_TOP (XEN_VIRT_END - PAGE_SIZE)
> > +/* This constant is derived from enum fixed_addresses_x below */
> > +#define MAX_FIXADDR_X_SIZE (2 << PAGE_SHIFT)
> 
> Answering slightly out of order, for clarity:
> 
> FIXADDR_X_SIZE should be 0 or 1 by the end of this patch.
> 
> As for MAX_FIXADDR_X_SIZE, how about simply
> IS_ENABLED(CONFIG_HYPERV_GUEST) ?  That should work, even in a linker
> script.
> 

It should be at least 1<<PAGE_SHIFT because __end_of_fixed_addresses_x
is at least 1.

So for now it can be

#define MAX_FIXADDR_X_SIZE ((IS_ENABLED(CONFIG_HYPERV_GUEST) + 1) << 
PAGE_SHIFT) 

> Somewhere, there should be a BUILD_BUG_ON() cross-checking
> MAX_FIXADDR_X_SIZE and __end_of_fixed_addresses_x.  We don't yet have a
> build_assertions() in x86/mm.c, so I guess now is the time to gain one.

No, the build_assertions shouldn't be necessary. 

MAX_FIXADDR_X_SIZE is intentionally set to the maximum possible value,
while __end_of_fixed_addresses_x is subject to change according to
configuration. They can differ.

Unless you're talking about adding CONFIG_HYPERV_GUEST to
MAX_FIXADDR_X_SIZE like I mentioned above?

> 
> >  
> >  #ifndef __ASSEMBLY__
> >  
> > @@ -89,6 +92,31 @@ static inline unsigned long virt_to_fix(const unsigned 
> > long vaddr)
> >      return __virt_to_fix(vaddr);
> >  }
> >  
> > +enum fixed_addresses_x {
> > +    /* Index 0 is reserved since fix_x_to_virt(0) == FIXADDR_X_TOP. */
> > +    FIX_X_RESERVED,
> > +#ifdef CONFIG_HYPERV_GUEST
> > +    FIX_X_HYPERV_HCALL,
> > +#endif
> > +    __end_of_fixed_addresses_x
> > +};
> > +
> > +#define FIXADDR_X_SIZE  (__end_of_fixed_addresses_x << PAGE_SHIFT)
> 
> -1, seeing as 0 is reserved.
> 

What does -1 mean here?

> > +#define FIXADDR_X_START (FIXADDR_X_TOP - FIXADDR_X_SIZE)
> > +
> > +extern void __set_fixmap_x(
> > +    enum fixed_addresses_x idx, unsigned long mfn, unsigned long flags);
> > +
> > +#define set_fixmap_x(idx, phys) \
> > +    __set_fixmap_x(idx, (phys)>>PAGE_SHIFT, PAGE_HYPERVISOR_RX | 
> > MAP_SMALL_PAGES)
> > +
> > +#define clear_fixmap_x(idx) __set_fixmap_x(idx, 0, 0)
> > +
> > +#define __fix_x_to_virt(x) (FIXADDR_X_TOP - ((x) << PAGE_SHIFT))
> > +#define __virt_to_fix_x(x) ((FIXADDR_X_TOP - ((x)&PAGE_MASK)) >> 
> > PAGE_SHIFT)
> 
> The &PAGE_MASK is redundant, given the following shift, but can't be
> optimised out because of its effect on the high 12 bits of the address
> as well.  These helpers aren't safe to wild inputs, even with the
> &PAGE_MASK, so I'd just drop it.
> 

OK. I will drop it.

Wei.

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