On 22/08/19 20:29, Laszlo Ersek wrote: > On 08/22/19 08:18, Paolo Bonzini wrote: >> On 21/08/19 22:17, Kinney, Michael D wrote: >>> DMA protection of memory ranges is a chipset feature. For the current >>> QEMU implementation, what ranges of memory are guaranteed to be >>> protected from DMA? Is it only A/B seg and TSEG? >> >> Yes. > > This thread (esp. Jiewen's and Mike's messages) are the first time that > I've heard about the *existence* of such RAM ranges / the chipset > feature. :) > > Out of interest (independently of virtualization), how is a general > purpose OS informed by the firmware, "never try to set up DMA to this > RAM area"? Is this communicated through ACPI _CRS perhaps? > > ... Ah, almost: ACPI 6.2 specifies _DMA, in "6.2.4 _DMA (Direct Memory > Access)". It writes, > > For example, if a platform implements a PCI bus that cannot access > all of physical memory, it has a _DMA object under that PCI bus that > describes the ranges of physical memory that can be accessed by > devices on that bus. > > Sorry about the digression, and also about being late to this thread, > continually -- I'm primarily following and learning.
It's much simpler: these ranges are not in e820, for example kernel: BIOS-e820: [mem 0x0000000000059000-0x000000000008bfff] usable kernel: BIOS-e820: [mem 0x000000000008c000-0x00000000000fffff] reserved The ranges are not special-cased in any way by QEMU. Simply, AB-segs and TSEG RAM are not part of the address space except when in SMM. Therefore, DMA to those ranges ends up respectively to low VGA RAM[1] and to the bit bucket. When AB-segs are open, for example, DMA to that area becomes possible. Paolo [1] old timers may remember DEF SEG=&HB800: BLOAD "foo.img",0. It still works with some disk device models. -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#46233): https://edk2.groups.io/g/devel/message/46233 Mute This Topic: https://groups.io/mt/32979681/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
