Re: tb_flush() calls causing long Windows XP boot times

[Mark Cave-Ayland]

On 16/06/2021 02:58, Richard Henderson wrote:

> On 6/15/21 6:58 AM, Programmingkid wrote:
> > > Ahh I misread - so those are the addresses of the routines and not where
> > > it's sticking the breakpoint?
> > >
> > > I notice from a bit of googling that there is a boot debugger. I wonder
> > > if /nodebug in boot.ini stops this behaviour?
> > >
> > >   
> > > https://docs.microsoft.com/en-us/troubleshoot/windows-server/performance/switch-options-for-boot-files 
> > >
> > >
> > > --
> > > Alex Bennée
> >
> > Hi Alex,
> >
> > I tried your suggestion of using /nodebug. It did not stop the tb_flush() function 
> > from being called.
>
> We are not expecting zero calls to tb_flush (it is used for other things, including 
> buffer full), but we are hoping that it reduces the frequency of the calls.
>
> I'm guessing you didn't immediately see the slowdown vanish, and so there was no 
> change to the frequency of the calls.
>
> FWIW, if you switch to the qemu console, you can see how many flushes have occurred 
> with "info jit".

Looking at the diff of b55f54bc96 which first introduced the regression, presumably 
the difference is now that everything is being flushed rather than a specific address 
space when WinXP twiddles with the DB7 register:


diff --git a/exec.c b/exec.c
index 67e520d18e..7f4074f95e 100644
--- a/exec.c
+++ b/exec.c
@@ -1019,14 +1019,13 @@ void tb_invalidate_phys_addr(AddressSpace *as, hwaddr addr, 
MemTxAttrs attrs)

 static void breakpoint_invalidate(CPUState *cpu, target_ulong pc)
 {
-    MemTxAttrs attrs;
-    hwaddr phys = cpu_get_phys_page_attrs_debug(cpu, pc, &attrs);
-    int asidx = cpu_asidx_from_attrs(cpu, attrs);
-    if (phys != -1) {
-        /* Locks grabbed by tb_invalidate_phys_addr */
-        tb_invalidate_phys_addr(cpu->cpu_ases[asidx].as,
-                                phys | (pc & ~TARGET_PAGE_MASK), attrs);
-    }
+    /*
+     * There may not be a virtual to physical translation for the pc
+     * right now, but there may exist cached TB for this pc.
+     * Flush the whole TB cache to force re-translation of such TBs.
+     * This is heavyweight, but we're debugging anyway.
+     */
+    tb_flush(cpu);
 }
 #endif


Unfortunately my x86-fu isn't really enough to understand what the solution should be 
in this case.


ATB,

Mark.