Hi, On Thu, Dec 27, 2018 at 9:32 PM Eduardo Habkost <ehabk...@redhat.com> wrote: > > On Fri, Dec 21, 2018 at 11:10:30AM -0500, Michael S. Tsirkin wrote: > > On Thu, Dec 06, 2018 at 10:32:13AM +0800, Li Zhijian wrote: > > > a new field xloadflags was added to recent x86 linux, and BIT 1: > > > XLF_CAN_BE_LOADED_ABOVE_4G is used to tell bootload that where initrd can > > > be > > > loaded safely. > > > > > > Current QEMU/BIOS always loads initrd below below_4g_mem_size which is > > > always > > > less than 4G, so here limiting initrd_max to 4G - 1 simply is enough if > > > this bit is set. > > > > > > CC: Paolo Bonzini <pbonz...@redhat.com> > > > CC: Richard Henderson <r...@twiddle.net> > > > CC: Eduardo Habkost <ehabk...@redhat.com> > > > CC: "Michael S. Tsirkin" <m...@redhat.com> > > > CC: Marcel Apfelbaum <marcel.apfelb...@gmail.com> > > > Signed-off-by: Li Zhijian <lizhij...@cn.fujitsu.com> > > > > > > --- > > > V3: correct grammar and check XLF_CAN_BE_LOADED_ABOVE_4G first (Michael > > > S. Tsirkin) > > > > > > Signed-off-by: Li Zhijian <lizhij...@cn.fujitsu.com> > > > --- > > > hw/i386/pc.c | 10 +++++++++- > > > 1 file changed, 9 insertions(+), 1 deletion(-) > > > > > > diff --git a/hw/i386/pc.c b/hw/i386/pc.c > > > index 3b10726..baa99c0 100644 > > > --- a/hw/i386/pc.c > > > +++ b/hw/i386/pc.c > > > @@ -904,7 +904,15 @@ static void load_linux(PCMachineState *pcms, > > > #endif > > > > > > /* highest address for loading the initrd */ > > > - if (protocol >= 0x203) { > > > + if (protocol >= 0x20c && > > > + lduw_p(header+0x236) & XLF_CAN_BE_LOADED_ABOVE_4G) { > > > + /* > > > + * Although kernel allows initrd loading to above 4G, > > > + * it just makes it as large as possible while still staying > > > below 4G > > > + * since current BIOS always loads initrd below > > > pcms->below_4g_mem_size > > > + */ > > > + initrd_max = UINT32_MAX; > > > + } else if (protocol >= 0x203) { > > > initrd_max = ldl_p(header+0x22c); > > > } else { > > > initrd_max = 0x37ffffff; > > > > > > I still have trouble understanding the above. > > Anyone else wants to comment / help rephrase the comment > > and commit log so it's readable? > > > The comment seems to contradict what I see on the code: > > | Although kernel allows initrd loading to above 4G, > > Sounds correct. > > > | it just makes it as large as possible while still staying below 4G > > I'm not a native English speaker, but I believe "it" here should > be interpreted as "the kernel", which would be incorrect. It's > this QEMU function that limits initrd_max to a uint32 value, not > the kernel. > > > | since current BIOS always loads initrd below pcms->below_4g_mem_size > > I don't know why the BIOS is mentioned here. The > below_4g_mem_size limit comes from these 2 lines inside > load_linux(): > > if (initrd_max >= pcms->below_4g_mem_size - pcmc->acpi_data_size) { > initrd_max = pcms->below_4g_mem_size - pcmc->acpi_data_size - 1; > } > In addition to that, initrd_max is uint32_t simply because QEMU > doesn't support the 64-bit boot protocol (specifically the > ext_ramdisk_image field), so all talk about below_4g_mem_size > seems to be just a distraction. > > All that said, I miss one piece of information here: is > XLF_CAN_BE_LOADED_ABOVE_4G really supposed to override > header+0x22c? linux/Documentation/x86/boot.txt isn't clear about > that. Is there any reference that can help us confirm this?
Looking at the following patch seems that we can confirm the assumption: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4bf7111f50167133a71c23530ca852a41355e739 Note: the patch was reverted due to bugs in some firmwares, but IMHO the assumption is correct. https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=47226ad4f4cfd1e91ded7f2ec42f83ff1c624663 Cheers, Stefano > > -- > Eduardo >