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Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg:
From: |
Gerd Hoffmann |
Subject: |
Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution" |
Date: |
Wed, 15 Mar 2017 16:25:39 +0100 |
Hi,
> Instead of having MMIO register spaces we use the dirty tracking
> mechanism. Here regions are marked as for dirty tracking and when the
> SoftMMU helper first comes to this bit of memory it will follow the slow
> path and mark region as visited.
visited? Do you mean dirty bit is set for the page? Or is this
something else?
> Once done this bit is cleared and all
> future writes to that page are written directly from the translated
> code. This no longer has an implicit synchronisation from the BQL so
> there is now a race and you can have memory being updated which might
> miss this flagging.
Not fully following what you are trying to say. But pages being updated
without dirty bit getting set (if clear) certainly is a problem for the
vga emulation (and live migration too).
> Note that KVM has some similar hacks to avoid trapping all writes to
> video memory with its coalesced mmio mechanism however I'm not familiar
> with all the details.
Normal linear framebuffer access doesn't use this.
> Now there are mechanisms we can use to ensure there are no races happen
> and return to the situation that the display is only updated when the
> TCG cores are not running.
tcg and display updates running in parallel isn't a problem, we have
that with kvm anyway. Dirty bit handling must be correct though.
With kvm at the start of each display update vga fetches the dirty
bitmap from the kernel (memory_region_sync_dirty_bitmap). Then it goes
use memory_region_get_dirty to figure which pages have been touched.
When memory_region_sync_dirty_bitmap is called the kernel will clear the
memory bitmap of the region and also map all pages read-only. Next
guest update will pagefault and the kernel can set the dirty bit for the
page (maybe there is a more efficient way with EPT available).
I suspect the memory_region_sync_dirty_bitmap call on tcg should reset
the fast path optimization, so the slow path can update the dirty bits
correctly.
cheers,
Gerd
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", (continued)
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Alex Bennée, 2017/03/14
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", BALATON Zoltan, 2017/03/14
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", luigi burdo, 2017/03/14
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Alex Bennée, 2017/03/15
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Mark Cave-Ayland, 2017/03/15
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Alex Bennée, 2017/03/15
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Paolo Bonzini, 2017/03/16
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Alex Bennée, 2017/03/16
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Paolo Bonzini, 2017/03/16
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Alex Bennée, 2017/03/15
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution",
Gerd Hoffmann <=
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Alex Bennée, 2017/03/15
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Gerd Hoffmann, 2017/03/16
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Alex Bennée, 2017/03/16
- Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution", Paolo Bonzini, 2017/03/16