Date: Tue, 28 Nov 2023 11:06:50 -0700 From: Warner Losh <imp@bsdimp.com> To: John Baldwin <jhb@freebsd.org> Cc: Warner Losh <imp@freebsd.org>, src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: c596126a5d3d - main - pmbr: Only load the first 545k rather than error out Message-ID: <CANCZdfr9eVA7bwSvzQo2_Vi%2B8edzqramDVboKyWCrqR9fQrXWA@mail.gmail.com> In-Reply-To: <e5973b11-f84c-40c7-bda1-dc666263eb3f@FreeBSD.org> References: <202311272246.3ARMkNS1037267@gitrepo.freebsd.org> <8fa6ed4d-c26b-4dd6-86f2-c3751f86d426@FreeBSD.org> <CANCZdfqC_Y2%2Bj7Vi5rWbKagyByk7xrWBy6H0q8qw8hDpZc4WuQ@mail.gmail.com> <e5973b11-f84c-40c7-bda1-dc666263eb3f@FreeBSD.org>
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[-- Attachment #1 --] On Tue, Nov 28, 2023 at 10:56 AM John Baldwin <jhb@freebsd.org> wrote: > On 11/28/23 9:07 AM, Warner Losh wrote: > > On Tue, Nov 28, 2023 at 9:06 AM John Baldwin <jhb@freebsd.org> wrote: > >> But also, can't you just leave a gap in the partitions to leave room > >> for a future ESP? It might not always be safe to load a full 545k > >> depending on if the firmware has reserved some of memory just below > >> 640k. 545k is an upper bound on how much can be loaded in terms of > >> 640k - the load address, but the practical limit might be lower. It is > >> probably safer as a general rule to keep the boot partition closer to > >> the size of the boot loader and leave a gap in place instead. > >> > > > > Today we load 512k always. > > Well, we load the partition size always. The default size of the partition > has grown over time. Arguably we should have been using padding instead of > growing the partition, but that is harder to write in installer tools, etc. > When I wrote pmbr I on purpose used the partition size as the size to load > rather than trying to assume information about the payload and parse > length from the payload due to the limited room in the MBR. I think we > started out creating freebsd-boot partitions that were 64k and later grew > them with a desire they could be replaced with an ESP partition, but ESPs > need a minimum size of 512k? > ESPs usually need at least a couple of megebytes since loader.efi is 660k right now and you really want a backup when you install a new one. It's better if it's tens of MB, though, since you may need it for other things... Warner [-- Attachment #2 --] <div dir="ltr"><div dir="ltr"><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Nov 28, 2023 at 10:56 AM John Baldwin <<a href="mailto:jhb@freebsd.org">jhb@freebsd.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">On 11/28/23 9:07 AM, Warner Losh wrote:<br> > On Tue, Nov 28, 2023 at 9:06 AM John Baldwin <<a href="mailto:jhb@freebsd.org" target="_blank">jhb@freebsd.org</a>> wrote:<br> >> But also, can't you just leave a gap in the partitions to leave room<br> >> for a future ESP? It might not always be safe to load a full 545k<br> >> depending on if the firmware has reserved some of memory just below<br> >> 640k. 545k is an upper bound on how much can be loaded in terms of<br> >> 640k - the load address, but the practical limit might be lower. It is<br> >> probably safer as a general rule to keep the boot partition closer to<br> >> the size of the boot loader and leave a gap in place instead.<br> >><br> > <br> > Today we load 512k always.<br> <br> Well, we load the partition size always. The default size of the partition<br> has grown over time. Arguably we should have been using padding instead of<br> growing the partition, but that is harder to write in installer tools, etc.<br> When I wrote pmbr I on purpose used the partition size as the size to load<br> rather than trying to assume information about the payload and parse<br> length from the payload due to the limited room in the MBR. I think we<br> started out creating freebsd-boot partitions that were 64k and later grew<br> them with a desire they could be replaced with an ESP partition, but ESPs<br> need a minimum size of 512k?<br></blockquote><div><br></div><div>ESPs usually need at least a couple of megebytes since loader.efi is 660k right now</div><div>and you really want a backup when you install a new one. It's better if it's tens of MB,</div><div>though, since you may need it for other things...</div><div><br></div><div>Warner</div></div></div>
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