On Wed, Oct 05, 2022 at 08:15:07AM +0200, Jan Beulich wrote: > On 04.10.2022 17:46, Demi Marie Obenour wrote: > > Linux has a function called efi_mem_reserve() that is used to reserve > > EfiBootServicesData memory that contains e.g. EFI configuration tables. > > This function does not work under Xen because Xen could have already > > clobbered the memory. efi_mem_reserve() not working is the whole reason > > for this thread, as it prevents EFI tables that are in > > EfiBootServicesData from being used under Xen. > > > > A much nicer approach would be for Xen to reserve boot services memory > > unconditionally, but provide a hypercall that dom0 could used to free > > the parts of EfiBootServicesData memory that are no longer needed. This > > would allow efi_mem_reserve() to work normally. > > efi_mem_reserve() actually working would be a layering violation; > controlling the EFI memory map is entirely Xen's job. Doing this properly would require Xen to understand all of the EFI tables that could validly be in EfiBootServices* and which could be of interest to dom0. It might (at least on some very buggy firmware) require a partial ACPI and/or SMBIOS implementation too, if the firmware decided to put an ACPI or SMBIOS table in EfiBootServices*. > As to the hypercall you suggest - I wouldn't mind its addition, but only > for the case when -mapbs is used. As I've indicated before, I'm of the > opinion that default behavior should be matching the intentions of the > spec, and the intention of EfiBootServices* is for the space to be > reclaimed. Plus I'm sure you realize there's a caveat with Dom0 using > that hypercall: It might use it for regions where data lives which it > wouldn't care about itself, but which an eventual kexec-ed (or alike) > entity would later want to consume. Code/data potentially usable by > _anyone_ between two resets of the system cannot legitimately be freed > (and hence imo is wrong to live in EfiBootServices* regions). I agree, but currently some such data *is* in EfiBootServices* regions, sadly. When -mapbs is *not* used, I recommend uninstalling all of the configuration tables that point to EfiBootServicesData memory before freeing that memory. > In a way one could view the Dom0 kernel as an "or alike" entity ... It is indeed such an entity. -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab
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